realpath() is failing to compile for me on MacOS 11.1 on an Apple M1 Mac. I encountered this when building GNU Global Tags from the most recent source archive at global-6.6.5.tar.gz <http://tamacom.com/global/global-6.6.5.tar.gz>.
This is due to an error because realpath cannot be found. The fix is to modify configure to include stdlib.h. Line 14291 of configure on global-6.6.5: main(){ (void)realpath("/./tmp", (void *)0); return 0; } Insert one line before that: #include<stdlib.h> main(){ (void)realpath("/./tmp", (void *)0); return 0; } configure now succeeds. The Brew folks and the Global Tags folks told me to report the bug here.