libphobos: Remove --enable-unix configure option.

libphobos: Remove --enable-unix configure option.

This option is not useful on its own as all posix modules require the
compiler to predefine version(Posix) anyway.  So the option has been
removed, and logic moved into DRUNTIME_OS_SOURCES, where the conditional
DRUNTIME_OS_POSIX is set instead.

libphobos/ChangeLog:

	* configure: Regenerate.
	* configure.ac: Remove DRUNTIME_OS_UNIX.
	* libdruntime/Makefile.am: Add DRUNTIME_DSOURCES_POSIX if
	DRUNTIME_OS_POSIX is true.
	* libdruntime/Makefile.in: Regenerate.
	* m4/druntime/os.m4 (DRUNTIME_OS_UNIX): Remove, move AM_CONDITIONAL
	logic to...
	(DRUNTIME_OS_SOURCES): ...here.  Rename conditional to
	DRUNTIME_OS_POSIX.