testsuite: stdlib/isomac.c: fix REQUIREMENTS

All of the mentioned variables are gone. gcc is just the default and
argv[1] can be used instead. /usr/include isn't hard-coded and you can
pass argv[2] with -I... to adjust.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>