configure.ac: Do not add content of $ac_cv_prog_cc_stdc to $CFLAGS as it's added to $CC automatically.

configure.ac

@@ -67,8 +67,6 @@ AC_PROG_LN_S
 AC_PROG_MAKE_SET
 AC_CANONICAL_HOST
 AC_PROG_CC_STDC
-AS_IF([[test -n "$ac_cv_prog_cc_stdc" && test "x$ac_cv_prog_cc_stdc" != "xno"]],
-      [[CFLAGS="$CFLAGS $ac_cv_prog_cc_stdc"]])
 LT_INIT([win32-dll])
 LT_LANG([Windows Resource])