libphobos: Remove --enable-druntime-gc configure option.

libphobos: Remove --enable-druntime-gc configure option.

This is yet another old option that would have been somewhat useful back
before the D front-end implementation was able to support compiling
without the Druntime library.

Now however, -fno-druntime makes the gcstub package redundant, so the
option has been removed, along with the package itself.

libphobos/ChangeLog:

	* configure: Regenerate.
	* libdruntime/Makefile.am (ALL_DRUNTIME_INSTALL_DSOURCES): Remove
	DRUNTIME_DSOURCES_GC and DRUNTIME_DSOURCES_GCSTUB.
	(DRUNTIME_DSOURCES): Add gc/*.d sources.
	(DRUNTIME_DSOURCES_GC): Remove.
	(DRUNTIME_DSOURCES_GCSTUB): Remove.
	* libdruntime/Makefile.in: Regenerate.
	* libdruntime/gcstub/gc.d: Remove.
	* m4/druntime.m4 (DRUNTIME_GC): Remove.