-Wwrite-strings: Remove -Wno-write-strings

-Wwrite-strings: Remove -Wno-write-strings

AFAIK GDB is now free from -Wwrite-strings warnings.  A few warnings may
be left behind in some host-specific code, but those should be few and
easy to fix.

gdb/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* warning.m4 (build_warnings): Remove -Wno-write-strings.
	* configure: Regenerate.

gdb/gdbserver/ChangeLog:
2017-04-05  Pedro Alves  <palves@redhat.com>

	* configure: Regenerate.