AC_PROG_CC_C99 is obsolete with autoconf >= 2.70 (#10383)

To make sure that compiler supports C99 before Autoconf 2.69, this was
needed. But with Autoconf 2.70 and later the macro is obsolete because
the checks are done in AC_PROG_CC and warnings are emitted when building
configure script.

Fixes part of GH-9483

Author: petk

configure.ac

@@ -128,7 +128,9 @@ PKG_PROG_PKG_CONFIG
 AC_PROG_CC([cc gcc])
 PHP_DETECT_ICC
 PHP_DETECT_SUNCC
-AC_PROG_CC_C99
+
+dnl AC_PROG_CC_C99 is obsolete with autoconf >= 2.70 yet necessary for <= 2.69.
+m4_version_prereq([2.70],,[AC_PROG_CC_C99])
 AC_PROG_CPP
 AC_USE_SYSTEM_EXTENSIONS
 AC_PROG_LN_S