libphobos: Add --with-libphobos-druntime-only option.

The intended purpose of the option is both for targets that don't
support phobos yet, and for gdc itself to support bootstrapping itself
as a self-hosted D compiler.

The libphobos testsuite has been updated to only add libphobos to the
search paths if it's being built.  A new D2 testsuite directive
RUNNABLE_PHOBOS_TEST has also been patched in to disable some runnable
tests that have phobos dependencies, of which is a temporary measure
until upstream DMD fixes or removes these tests entirely.

gcc/testsuite/ChangeLog:

	* lib/gdc-utils.exp (gdc-convert-test): Add dg-skip-if for tests that
	depending on the phobos standard library.

libphobos/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Add --with-libphobos-druntime-only option and the
	conditional ENABLE_LIBDRUNTIME_ONLY.
	* configure.tgt: Define LIBDRUNTIME_ONLY.
	* src/Makefile.am: Add phobos sources if not ENABLE_LIBDRUNTIME_ONLY.
	* src/Makefile.in: Regenerate.
	* testsuite/testsuite_flags.in: Add phobos path if compiling phobos.