shared: avoid checking for __STDC_VERSION__

shared: avoid checking for __STDC_VERSION__

Since we pass -std=gnu99 to the compiler __STDC_VERSION__ is defined to
199901. Even in this case we can use _Noreturn. Delegate the testing to
the configure phase with a compile test.

In future we want to stop using AC_PROG_CC_99 and start using the new
AC_PROG_CC from autoconf 2.70, which tries to choose the most recent
standard.