m4: follow GMP changes for GMP_PROG_CC_FOR_BUILD
GMP has been using
return 0; instead of
exit(0); for GMP_PROG_CC* m4 macros since 2014.
The current code for
conftest.c is technically incorrect, as
stdlib.h to be included:
This commit fixes the case where the compiler expects
#include <stdlib.h> to use
exit, which was observed to be an issue in some environnements.