Don't always assume that `__STDC_VERSION__` is defined
Hi,
Ref.: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1102782
gperf currently generates code that always assumes __STDC_VERSION__
is defined, which is not correct.
This MR backports the upstream fix from https://git.savannah.gnu.org/gitweb/?p=gperf.git;a=commit;h=3bab6ab7bff26bb444722ff9fc1c58b523e9262d. I tested it against the kcodecs
package (which currently FTBFSes because of this issue), and it works as expected.
Thanks!