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.