Let us properly override with CMAKE_BUILD_TYPE=Gentoo

--- a/CMakeLists.txt	2018-12-11 18:57:55.000000000 +0100
+++ b/CMakeLists.txt	2020-07-21 16:43:33.467680534 +0200
@@ -14,18 +14,7 @@
 set(Eigen_BINARY_DIR ${Eigen3_BINARY_DIR})
 
 # guard against bad build-type strings
-
-if (NOT CMAKE_BUILD_TYPE)
-  set(CMAKE_BUILD_TYPE "Release")
-endif()
-
-string(TOLOWER "${CMAKE_BUILD_TYPE}" cmake_build_type_tolower)
-if(    NOT cmake_build_type_tolower STREQUAL "debug"
-   AND NOT cmake_build_type_tolower STREQUAL "release"
-   AND NOT cmake_build_type_tolower STREQUAL "relwithdebinfo")
-  message(FATAL_ERROR "Unknown build type \"${CMAKE_BUILD_TYPE}\". Allowed values are Debug, Release, RelWithDebInfo (case-insensitive).")
-endif()
-
+# yeah no.
 
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 # retrieve version infomation                                               #