The normal process of building a cross compilation toolchain is:

  1. build GCC without the standard C lib
  2. build the standard C lib
  3. rebuild GCC
  4. rebuild the standard C lib with the new GCC

But for AVR, GCC can be build successfully without (a provisionally version of)
the standard C headers available - due to the lack of libstdc++ support
for AVR. But we still need to remove the check to allow for this.

diff --color -rupN a/gcc/Makefile.in b/gcc/Makefile.in
--- a/gcc/Makefile.in	2024-05-07 08:51:37.000000000 +0200
+++ b/gcc/Makefile.in	2024-05-09 11:55:59.115578602 +0200
@@ -3415,18 +3415,6 @@ stmp-fixinc: gsyslimits.h macro_list fix
 	    sysroot_headers_suffix=`echo $${ml} | sed -e 's/;.*$$//'`; \
 	    multi_dir=`echo $${ml} | sed -e 's/^[^;]*;//'`; \
 	    fix_dir=include-fixed$${multi_dir}; \
-	    if ! $(inhibit_libc) && test ! -d ${BUILD_SYSTEM_HEADER_DIR}; then \
-	      echo "The directory (BUILD_SYSTEM_HEADER_DIR) that should contain system headers does not exist:" >&2 ; \
-	      echo "  ${BUILD_SYSTEM_HEADER_DIR}" >&2 ; \
-	      case ${build_os} in \
-	        darwin*) \
-	          echo "(on Darwin this usually means you need to pass the --with-sysroot= flag to point to a valid MacOS SDK)" >&2; \
-	          ;; \
-	      esac; \
-	      tooldir_sysinc=`echo "${gcc_tooldir}/sys-include" | sed -e :a -e "s,[^/]*/\.\.\/,," -e ta`; \
-	      if test "x${BUILD_SYSTEM_HEADER_DIR}" = "x$${tooldir_sysinc}"; \
-	      then sleep 1; else exit 1; fi; \
-	    fi; \
 	    $(mkinstalldirs) $${fix_dir}; \
 	    chmod a+rx $${fix_dir} || true; \
 	    (TARGET_MACHINE='$(target)'; srcdir=`cd $(srcdir); ${PWD_COMMAND}`; \