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.