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 exit(int) requires stdlib.h to be included: https://www.man7.org/linux/man-pages/man3/exit.3.html

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.