We definitely do not want to replace system headers.

Index: src/makefile
--- src/makefile.orig
+++ src/makefile
@@ -186,7 +186,6 @@ chkccomp.h:
 	echo "#define CC_OPT_LINK_TIME_OPTIMIZATION \"-flto\"" >> chkccomp.h
 	echo "#define LINKER_OPT_NO_LTO \"-fno-lto\"" >> chkccomp.h
 	echo "#define SUPPORTS_PARTIAL_LINKING" >> chkccomp.h
-	echo "#define ALLOW_REPLACEMENT_OF_SYSTEM_HEADERS" >> chkccomp.h
 
 base.h:
 	echo "#define PATH_DELIMITER '/'" > base.h