Enable libphobos on Solaris 11/x86

Enable libphobos on Solaris 11/x86

	toplevel:
	* configure.ac (enable_libphobos): Check LIBPHOBOS_SUPPORTED.
	* configure: Regenerate.

	libphobos:
	* configure.tgt (LIBPHOBOS_SUPPORTED): Default to no.
	Set to yes explicitly.
	* configure.ac: Handle --enable-libphobos.
	(x86_64-*-solaris2.* | i?86-*-solaris2.*): Only mark supported
	with gas.
	(ENABLE_LIBPHOBOS): New conditional.
	* configure: Regenerate.
	* Makefile.am (SUBDIRS): Only set if ENABLE_LIBPHOBOS.
	* Makefile.in: Regenerate.

From-SVN: r270348