CMake: set newer POSIX_C_SOURCE

[thesamesam]

Enable POSIX 2008 as it's needed for strdup(), otherwise we get:

/var/tmp/portage/sys-libs/minizip-ng-3.0.9/work/minizip-ng-3.0.9/mz_os.c: In function ‘mz_dir_make’:
/var/tmp/portage/sys-libs/minizip-ng-3.0.9/work/minizip-ng-3.0.9/mz_os.c:286:19: error: implicit declaration of function ‘strdup’ [-Werror=implicit-function-declaration]
  286 |     current_dir = strdup(path);
      |                   ^~~~~~

The man page for strdup says:

strdup():
	_XOPEN_SOURCE >= 500
	|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
	|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

[thesamesam]

Thanks!