build/clang: fix detection of valid warning compiler flags

clang does not exit with error when it is called with an
unrecognized (warning) option. Instead it just prints
a warning that makes the configure script believe the
warning is supported. Later, during build we might pass
-Werror, which causes clang to fail due to unrecognized
arguments.

Fix the script to detect compiler warnings by passing
'-Werror=unknown-warning-option', which lets clang fail too.

Signed-off-by: Thomas Haller <thaller@redhat.com>

m4/compiler_warnings.m4

@@ -7,6 +7,23 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 	AC_MSG_RESULT(yes)
 	CFLAGS="-Wall -std=gnu89 $CFLAGS"
 
+	dnl clang only warns about unknown warnings, unless
+	dnl called with "-Werror=unknown-warning-option"
+	dnl Test if the compiler supports that, and if it does
+	dnl attach it to the CFLAGS.
+	SAVE_CFLAGS="$CFLAGS"
+	EXTRA_CFLAGS="-Werror=unknown-warning-option"
+	CFLAGS="$SAVE_CFLAGS $EXTRA_CFLAGS"
+	AC_TRY_COMPILE([], [],
+		has_option=yes,
+		has_option=no,)
+	if test $has_option = no; then
+		EXTRA_CFLAGS=
+	fi
+	CFLAGS="$SAVE_CFLAGS"
+	unset has_option
+	unset SAVE_CFLAGS
+
 	for option in -Wshadow -Wmissing-declarations -Wmissing-prototypes \
 		      -Wdeclaration-after-statement -Wformat-security \
 		      -Wfloat-equal -Wno-unused-parameter -Wno-sign-compare \
@@ -15,19 +32,22 @@ if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
 		      -Wpointer-arith -Winit-self \
 		      -Wmissing-include-dirs -Waggregate-return; do
 		SAVE_CFLAGS="$CFLAGS"
-		CFLAGS="$CFLAGS $option"
+		CFLAGS="$CFLAGS $EXTRA_CFLAGS $option"
 		AC_MSG_CHECKING([whether gcc understands $option])
 		AC_TRY_COMPILE([], [],
 			has_option=yes,
 			has_option=no,)
 		if test $has_option = no; then
 			CFLAGS="$SAVE_CFLAGS"
+		else
+			CFLAGS="$SAVE_CFLAGS $option"
 		fi
 		AC_MSG_RESULT($has_option)
 		unset has_option
 		unset SAVE_CFLAGS
 	done
 	unset option
+	unset EXTRA_CFLAGS
 	if test "x$set_more_warnings" = xerror; then
 		CFLAGS="$CFLAGS -Werror"
 	fi