Fix -Wno-unknown-warning support detection

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.