Consistently use block comments at beginning of C99 and C11 tests

Consistently use block comments at beginning of C99 and C11 tests

Purely cosmetic improvement to the C standard version tests: Use /* */
comments above all three checks of __STDC__ and/or __STDC_VERSION__,
and make “// See if C++-style comments work” the very first // comment.

Resolves issue #110713.

* c.m4 (_AC_C_C99_TEST_GLOBALS, _AC_C_C11_TEST_GLOBALS):
  Improve stylistic consistency with _AC_C_C89_TEST_GLOBALS.
  Move explicit test of C++-style comments earlier.