* Makefile.in: Add support for building shared libraries under Darwin.
(EXTRA_GNATRTL_NONTASKING_OBJS, ppc-vxworks): Add s-vxwexc.o, containing
the low level EH init subprogram to be called from __gnat_initialize.
* mlib-tgt.ads, mlib-tgt.adb (Archive_Indexer_Options): New subprogram,
indicates which options to pass to the archive indexer.
* mlib-utl.adb: Add support for calling ranlib with additional
options. This is needed for instance under Mac OS X.
(Ranlib_Options): New global variable, used to store the potential
options to pass to ranlib.
(Ar): Use Ranlib_Options when spawning ranlib.
(Initialize): Set the value of ranlib option.