Fix -Wno-unknown-warning support detection

Ref: https://sourceware.org/ml/gdb/2015-12/msg00024.html

We have code in configure.ac that tries to detect whether the compiler
supports each warning and suppress it if not, but that doesn't work
with "-Wno-" options, because gcc doesn't error out for
-Wno-unknown-warning unless other diagnostics are being produced.

See https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html.

Handle this by checking whether -Wfoo works when we actually want
-Wno-foo.

gdb/ChangeLog:
2015-12-16  Pedro Alves  <palves@redhat.com>

	* configure.ac (compiler warning flags): When testing a
	-Wno-foo option, check whether -Wfoo works instead.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2015-12-16  Pedro Alves  <palves@redhat.com>

	* configure.ac (compiler warning flags): When testing a
	-Wno-foo option, check whether -Wfoo works instead.
	* configure: Regenerate.

gdb/gdbserver/configure

@@ -6148,14 +6148,23 @@ $as_echo_n "checking compiler warning flags... " >&6; }
     # Separate out the -Werror flag as some files just cannot be
     # compiled with it enabled.
     for w in ${build_warnings}; do
+	# GCC does not complain about -Wno-unknown-warning.  Invert
+	# and test -Wunknown-warning instead.
+	case $w in
+	-Wno-*)
+		wtest=`echo $w | sed 's/-Wno-/-W/g'` ;;
+	*)
+		wtest=$w ;;
+	esac
+
 	case $w in
 	-Werr*) WERROR_CFLAGS=-Werror ;;
 	*)
 	    # Check whether GCC accepts it.
 	    saved_CFLAGS="$CFLAGS"
-	    CFLAGS="$CFLAGS $w"
+	    CFLAGS="$CFLAGS $wtest"
 	    saved_CXXFLAGS="$CXXFLAGS"
-	    CXXFLAGS="$CXXFLAGS $w"
+	    CXXFLAGS="$CXXFLAGS $wtest"
 	    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 /* end confdefs.h.  */