enable -Wwrite-strings for gas

enable -Wwrite-strings for gas

We add a new AC_SUBST to warning.m4 so that the test if the warning is
supported is centralized, but the warning can be enabled per directory.

binutils/ChangeLog:

2016-03-31  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* configure: Regenerate.

gprof/ChangeLog:

2016-03-31  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* configure: Regenerate.

ld/ChangeLog:

2016-03-31  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* configure: Regenerate.

opcodes/ChangeLog:

2016-03-31  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* configure: Regenerate.

bfd/ChangeLog:

2016-03-31  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* configure: Regenerate.
	* warning.m4: Add WARN_WRITE_STRINGS AC_SUBST.

gold/ChangeLog:

2016-03-31  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* configure: Regenerate.

gas/ChangeLog:

2016-03-31  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* Makefile.am: Add WARN_WRITE_STRINGS to WARN_CFLAGS.
	* Makefile.in: Regenerate.
	* configure: Likewise.