Partly fixed a clang -Wreserved-id-macro warning with _GNU_SOURCE
Partly fixed a clang -Wreserved-id-macro warning with _GNU_SOURCE

_GNU_SOURCE is only defined for the syscall() function, so only define it if HAVE_SYSCALL_GETRANDOM is also defined.

This fixes the only remaining -Wreserved-id-macro, at least for many cases (like macOS, Windows, and BSD).

libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat