Enable C99 in autoconf

This is essentially a no-op, because any current compiler already
defaults to C99 or C11.

When using language features from C99 or C11, please don't forget
that header files also need to be compatible with C++.

Author: nikic

configure.ac

@@ -183,8 +183,7 @@ AC_PROG_CC([cc gcc])
 PHP_DETECT_ICC
 PHP_DETECT_SUNCC
 AC_PROG_CC_C_O
-dnl Change to AC_PROG_CC_STDC when we start requiring a post-2.13 autoconf
-dnl AC_PROG_CC_STDC
+AC_PROG_CC_C99
 AC_PROG_CPP
 AC_USE_SYSTEM_EXTENSIONS
 AC_PROG_LN_S