--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -141,12 +141,16 @@
     MESSAGE(status "Skipping compiler version check.")
 ENDIF()
 
-# Configure C++ standard if not externally specified (will actually be
-# set after CppUnit check below). Use the variable CMAKE_CXX_STANDARD
-# since it will actually be used for this purposes starting in CMake 3.1.
+########################################################################
+# Configure C++ standard if not externally specified. Use the variable
+# CMAKE_CXX_STANDARD since it will actually be used for this purposes
+# starting in CMake 3.1.
+########################################################################
 
-SET(CMAKE_C_EXTENSIONS OFF)
-SET(CMAKE_CXX_EXTENSIONS OFF)
+# save current setting, if provided
+IF(CMAKE_CXX_STANDARD)
+    SET(CMAKE_CXX_STANDARD_EXT ${CMAKE_CXX_STANDARD})
+ENDIF()
 
 IF(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     SET(CMAKE_CXX_STANDARD 11)
@@ -155,9 +159,60 @@
 ELSEIF(CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
     SET(CMAKE_CXX_STANDARD 11)
 ELSE()
-    message(WARNING "C++ standard could not be set because compiler is not GNU, Clang or MSVC.")
+    MESSAGE(WARNING "C++ standard could not be set because compiler is not GNU, Clang or MSVC.")
+ENDIF()
+
+# if the CXX standard was provided externally, make sure it meets GR's minimum
+IF(CMAKE_CXX_STANDARD_EXT)
+    # verify that the provided C++ standard is one CMake
+    # accepts. options are 90, 99, and 11 as of CMake 3.11.4 (with 20
+    # coming in 3.12).
+    IF(NOT ${CMAKE_CXX_STANDARD_EXT} VERSION_EQUAL "98" AND
+       NOT ${CMAKE_CXX_STANDARD_EXT} VERSION_EQUAL "11" AND
+       NOT ${CMAKE_CXX_STANDARD_EXT} VERSION_EQUAL "14" AND
+       NOT ${CMAKE_CXX_STANDARD_EXT} VERSION_EQUAL "17")
+       MESSAGE(WARNING "Requested C++ standard (${CMAKE_CXX_STANDARD_EXT}) isn't recognized by CMake; expecting one of 98, 11, 14, or 17. Ignoring and using the GNU Radio default C++ standard (${CMAKE_CXX_STANDARD}).")
+       SET(CMAKE_CXX_STANDARD_EXT ${CMAKE_CXX_STANDARD})
+    ENDIF()
+    # fix settings to be comparables. It will be -many- years until we
+    # have to worry about this value getting large enough to wrap
+    # around (to 90), so just add 100 to any value not in the 90's,
+    # for comparison sake.
+    IF(${CMAKE_CXX_STANDARD} VERSION_EQUAL "98")
+        # just copy the value
+        SET(CMAKE_CXX_STANDARD_CMP ${CMAKE_CXX_STANDARD})
+    ELSE()
+        # add 100 to the value
+        MATH(EXPR CMAKE_CXX_STANDARD_CMP "${CMAKE_CXX_STANDARD} + 100")
+    ENDIF()
+    IF(${CMAKE_CXX_STANDARD_EXT} VERSION_EQUAL "98")
+        # just copy the value
+        SET(CMAKE_CXX_STANDARD_EXT_CMP ${CMAKE_CXX_STANDARD_EXT})
+    ELSE()
+        # add 100 to the value
+        MATH(EXPR CMAKE_CXX_STANDARD_EXT_CMP "${CMAKE_CXX_STANDARD_EXT} + 100")
+    ENDIF()
+    # compare values
+    IF(${CMAKE_CXX_STANDARD_EXT_CMP} VERSION_LESS ${CMAKE_CXX_STANDARD_CMP})
+        MESSAGE(WARNING "Requested C++ standard (${CMAKE_CXX_STANDARD_EXT}) isn't modern enough to build GNU Radio; using minimum required C++ standard (${CMAKE_CXX_STANDARD}) instead and hoping for the best!")
+    ELSEIF(${CMAKE_CXX_STANDARD_EXT_CMP} VERSION_GREATER ${CMAKE_CXX_STANDARD_CMP})
+        MESSAGE(WARNING "Requested C++ standard (${CMAKE_CXX_STANDARD_EXT}) is more recent than the minimum required C++ standard (${CMAKE_CXX_STANDARD}). GNU Radio might have issues building using this C++ standard; going with it and hoping for the best!")
+        SET(CMAKE_CXX_STANDARD ${CMAKE_CXX_STANDARD_EXT})
+    ENDIF()
+ENDIF()
+
+########################################################################
+# Configure C standard if not externally specified. Use the variable
+# CMAKE_C_STANDARD since it will actually be used for this purposes
+# starting in CMake 3.1.
+########################################################################
+
+# save current setting, if provided
+IF(CMAKE_C_STANDARD)
+    SET(CMAKE_C_STANDARD_EXT ${CMAKE_C_STANDARD})
 ENDIF()
 
+# set GR preferred setting
 IF(CMAKE_C_COMPILER_ID STREQUAL "GNU")
     SET(CMAKE_C_STANDARD 11)
 ELSEIF(CMAKE_C_COMPILER_ID MATCHES "Clang")
@@ -165,7 +220,58 @@
 ELSEIF(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
     SET(CMAKE_C_STANDARD 11)
 ELSE()
-    message(WARNING "C standard could not be set because compiler is not GNU, Clang or MSVC.")
+    MESSAGE(WARNING "C standard could not be set because compiler is not GNU, Clang or MSVC.")
+ENDIF()
+
+# if the C standard was provided externally, make sure it meets GR's minimum
+IF(CMAKE_C_STANDARD_EXT)
+    # verify that the provided C standard is one CMake
+    # accepts. options are 90, 99, and 11 as of CMake 3.11.4.
+    IF(NOT ${CMAKE_C_STANDARD_EXT} VERSION_EQUAL "90" AND
+       NOT ${CMAKE_C_STANDARD_EXT} VERSION_EQUAL "99" AND
+       NOT ${CMAKE_C_STANDARD_EXT} VERSION_EQUAL "11")
+       MESSAGE(WARNING "Requested C standard (${CMAKE_C_STANDARD_EXT}) isn't recognized by CMake; expecting one of 90, 99, or 11. Ignoring and using the GNU Radio default C standard (${CMAKE_C_STANDARD}).")
+       SET(CMAKE_C_STANDARD_EXT ${CMAKE_C_STANDARD})
+    ENDIF()
+    # fix settings to be comparable. It will be -many- years until we
+    # have to worry about this value getting large enough to wrap
+    # around (to 90), so just add 100 to any value not in the 90's,
+    # for comparison sake.
+    IF(${CMAKE_C_STANDARD} VERSION_EQUAL "90" OR
+       ${CMAKE_C_STANDARD} VERSION_EQUAL "99")
+        # just copy the value
+        SET(CMAKE_C_STANDARD_CMP ${CMAKE_C_STANDARD})
+    ELSE()
+        # add 100 to the value
+        MATH(EXPR CMAKE_C_STANDARD_CMP "${CMAKE_C_STANDARD} + 100")
+    ENDIF()
+    IF(${CMAKE_C_STANDARD_EXT} VERSION_EQUAL "90" OR
+       ${CMAKE_C_STANDARD_EXT} VERSION_EQUAL "99")
+        # just copy the value
+        SET(CMAKE_C_STANDARD_EXT_CMP ${CMAKE_C_STANDARD_EXT})
+    ELSE()
+        # add 100 to the value
+        MATH(EXPR CMAKE_C_STANDARD_EXT_CMP "${CMAKE_C_STANDARD_EXT} + 100")
+    ENDIF()
+    # compare values
+    IF(${CMAKE_C_STANDARD_EXT_CMP} VERSION_LESS ${CMAKE_C_STANDARD_CMP})
+        MESSAGE(WARNING "Requested C standard (${CMAKE_C_STANDARD_EXT}) isn't modern enough to build GNU Radio; using minimum required C standard (${CMAKE_C_STANDARD}) instead and hoping for the best!")
+    ELSEIF(${CMAKE_C_STANDARD_EXT_CMP} VERSION_GREATER ${CMAKE_C_STANDARD_CMP})
+        MESSAGE(WARNING "Requested C standard (${CMAKE_C_STANDARD_EXT}) is more recent than the minimum required C standard (${CMAKE_C_STANDARD}). GNU Radio might have issues building using this C standard; going with it and hoping for the best!")
+        SET(CMAKE_C_STANDARD ${CMAKE_C_STANDARD_EXT})
+    ENDIF()
+ENDIF()
+
+########################################################################
+# If C/CXX EXTENSIONS are defined, don't change them. GNU Radio itself
+# does not require them. If not defined, then disable them as a safe default.
+########################################################################
+
+IF(NOT CMAKE_C_EXTENSIONS)
+    SET(CMAKE_C_EXTENSIONS OFF)
+ENDIF()
+IF(NOT CMAKE_CXX_EXTENSIONS)
+    SET(CMAKE_CXX_EXTENSIONS OFF)
 ENDIF()
 
 ########################################################################