acinclude: Improve detection of not-understood compiler options with clang.

By default, clang warns about but does not fail on unknown -W options.
This made configure add the option to WARNING_FLAGS, which caused the
warning about not-understood warnings to be emitted for every file
compiled.

In combination with -Werror, clang does fail on unknown -W options.  This
commit adds -Werror during configure's warning tests, which should cause
the not-understood warnings to be detected that way.

Reported-by: Ed Maste <emaste@freebsd.org>
Tested-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>

Author: blp

acinclude.m4

@@ -362,15 +362,15 @@ dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
 dnl with or without modifications, as long as this notice is preserved.
 
-dnl OVS_CHECK_CC_OPTION([OPTION], [ACTION-IF-ACCEPTED], [ACTION-IF-REJECTED])
-dnl Check whether the given C compiler OPTION is accepted.
-dnl If so, execute ACTION-IF-ACCEPTED, otherwise ACTION-IF-REJECTED.
-AC_DEFUN([OVS_CHECK_CC_OPTION],
-[
+AC_DEFUN([_OVS_CHECK_CC_OPTION], [dnl
   m4_define([ovs_cv_name], [ovs_cv_[]m4_translit([$1], [-], [_])])dnl
   AC_CACHE_CHECK([whether $CC accepts $1], [ovs_cv_name], 
     [ovs_save_CFLAGS="$CFLAGS"
-     CFLAGS="$CFLAGS $1"
+     dnl Include -Werror in the compiler options, because without -Werror
+     dnl clang's GCC-compatible compiler driver does not return a failure
+     dnl exit status even though it complains about options it does not
+     dnl understand.
+     CFLAGS="$CFLAGS $WERROR $1"
      AC_COMPILE_IFELSE([AC_LANG_PROGRAM(,)], [ovs_cv_name[]=yes], [ovs_cv_name[]=no])
      CFLAGS="$ovs_save_CFLAGS"])
   if test $ovs_cv_name = yes; then
@@ -380,6 +380,21 @@ AC_DEFUN([OVS_CHECK_CC_OPTION],
   fi
 ])
 
+dnl OVS_CHECK_WERROR
+dnl
+dnl Check whether the C compiler accepts -Werror.
+dnl Sets $WERROR to "-Werror", if so, and otherwise to the empty string.
+AC_DEFUN([OVS_CHECK_WERROR],
+  [WERROR=
+   _OVS_CHECK_CC_OPTION([-Werror], [WERROR=-Werror])])
+
+dnl OVS_CHECK_CC_OPTION([OPTION], [ACTION-IF-ACCEPTED], [ACTION-IF-REJECTED])
+dnl Check whether the given C compiler OPTION is accepted.
+dnl If so, execute ACTION-IF-ACCEPTED, otherwise ACTION-IF-REJECTED.
+AC_DEFUN([OVS_CHECK_CC_OPTION],
+  [AC_REQUIRE([OVS_CHECK_WERROR])
+   _OVS_CHECK_CC_OPTION([$1], [$2], [$3])])
+
 dnl OVS_ENABLE_OPTION([OPTION])
 dnl Check whether the given C compiler OPTION is accepted.
 dnl If so, add it to WARNING_FLAGS.