Add -Wunused-variable to warnings.m4

Add -Wunused-variable to warnings.m4

This adds -Wunused-variable to the build.  This required a special
check in configure in order to work around a bug in GCC 4.9.  Simon
ound the correct test to use, so I've added him to the ChangeLog.

gdb/ChangeLog
2018-07-22  Simon Marchi  <simon.marchi@polymtl.ca>
	    Tom Tromey  <tom@tromey.com>

	* warning.m4 (AM_GDB_WARNINGS): Add -Wunused-variable and special
	test for it.
	* configure: Rebuild.

gdb/gdbserver/ChangeLog
2018-07-22  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.