https://cgit.openembedded.org/meta-openembedded/plain/meta-oe/recipes-devtools/mcpp/files/0001-configure-Fix-checks-for-system-headers.patch?id=6721430ca97aa98208cc1c450e4e8c4c274ca840

From c1e9f2f3d086e0df3c10a2468fd7b37fd0c5038c Mon Sep 17 00:00:00 2001
From: Khem Raj <raj.khem@gmail.com>
Date: Wed, 7 Sep 2022 00:02:08 -0700
Subject: [PATCH] configure: Fix checks for system headers

Define _DEFAULT_SOURCE in system.c so unistd.h can expose readlink API

Upstream-Status: Pending

Signed-off-by: Khem Raj <raj.khem@gmail.com>
--- a/configure.ac
+++ b/configure.ac
@@ -152,7 +152,7 @@ fi
 
 dnl Checks for header files.
 
-AC_CHECK_HEADERS( [unistd.h, stdint.h, inttypes.h])
+AC_CHECK_HEADERS( [unistd.h stdint.h inttypes.h])
 
 dnl Checks for typedefs, and compiler characteristics.
 
--- a/src/system.c
+++ b/src/system.c
@@ -36,6 +36,8 @@
  *      1. specify the constants in "configed.H" or "noconfig.H",
  *      2. append the system-dependent routines in this file.
  */
+
+#define _DEFAULT_SOURCE
 #if PREPROCESSED
 #include    "mcpp.H"
 #else