gdbsupport: use AM_GDB_WARNINGS

Since gdbsupport has been given its own build system, it is no longer
compiled with the warning flags specified in gdb/warning.m4.

This patch makes it use AM_GDB_WARNINGS.

gdbsupport/ChangeLog:

	* acinclude.m4: Include ../gdb/warning.m4.
	* configure.ac: Use AM_GDB_WARNINGS.
	* Makefile.am: Set AM_CFLAGS to WARN_CFLAGS and WERROR_CFLAGS.
	* Makefile.in: Re-generate.
	* configure: Re-generate.