eh_alloc.cc: Ensure that required macros are defined before including gthr.h.

	* libsupc++/eh_alloc.cc: Ensure that required macros are
	defined before including gthr.h.  Ensure that we get the
	version of gthr.h for which we know how to provide a
	configuration.
	* libsupc++/eh_globals.cc: Likewise.  And, bring the threading
	code path into line with the current EH model.  Use std, where
	appropriate.

Co-Authored-By: John David Anglin <dave@hiauly1.hia.nrc.ca>

From-SVN: r42999