Explicitly require c99 support

This is not a new requirement: we already needed c99 support for certain
functions (e.g., va_copy()).  However, gcc doesn't error out if functions
introduced in newer versions of the C standard are accessed when the standard
is something older.

configure.ac

@@ -6,12 +6,13 @@ AC_INIT(libverto, 0.2.6)
 m4_ifdef([AC_USE_SYSTEM_EXTENSIONS],
     [AC_USE_SYSTEM_EXTENSIONS],
     [AC_GNU_SOURCE])
+
+AC_PROG_CC_C99
 CFLAGS="$CFLAGS -Wall -Wextra"
 
 AC_CANONICAL_SYSTEM
 AM_INIT_AUTOMAKE([1.11])
 
-AC_PROG_CC
 AC_PROG_LIBTOOL
 AC_CONFIG_MACRO_DIR([m4])