Add checks of __STDC__ and __STDC_VERSION__ to C conformance tests.

Add checks of __STDC__ and __STDC_VERSION__ to C conformance tests.

This makes the C conformance tests more consistent with the C++
conformance tests, and should also speed up cycling through the
possible options to turn on C99/C11.

Tested with gcc, clang, SunPRO C, and AIX xlc.

* lib/autoconf/c.m4 (_AC_C_C89_TEST_GLOBALS): Add preprocessor test
  for __STDC__ being defined (to any value).
  (_AC_C_C99_TEST_GLOBALS, _AC_C_C11_TEST_GLOBALS): Add preprocessor
  test of the value of __STDC_VERSION__.