Make -Wformat-nonliteral work with gcc

Make -Wformat-nonliteral work with gcc

After looking into why the build failed for Simon but not for me, we
found that the underlying cause was due to how gcc treats
-Wformat-nonliteral.  gcc requires -Wformat to be given first; but
warning.m4 was not doing this, so -Wformat-nonliteral was not being
used.

This patch changes warning.m4 to account gcc's requirement.

This then showed that the target-float.c build change in the earlier
Makefile patch was also incorrect.  Simon didn't see this in his
build, but gcc now points it out.  So, this patch fixes this problem
as well.

2018-09-05  Tom Tromey  <tom@tromey.com>

	* warning.m4 (AM_GDB_WARNINGS): Add -Wformat when testing
	-Wformat-nonliteral.
	* target-float.c (host_float_ops<T>::to_string)
	(host_float_ops<T>::from_string): Use
	DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL.
	* configure: Rebuild.

gdb/gdbserver/ChangeLog
2018-09-05  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.