Hacks to get ODoors to build on Linux (Debian/Ubuntu recent releases) again

1. Resolve the "hidden symbol 'atexit' ... being referenced by DSO" error
   by using the -shared option (link against shared objects, not static libs)
2. The path to the xpdev output directory (where to find libxpdev.a) is just
   wrong. Hard-coded the machine architecuture portion (.x64) as a hack for
   now. Also, since this is use the LD macro, you need to pass "LD=gcc" on the
   make command-line. Not sure how this worked before.

Fixes (or at least works-around) issues reported by Ryan Fantus (1:218/820)

src/odoors/GNUmakefile

@@ -68,7 +68,7 @@ endif
 #
 CFLAGS	+=	-fPIC
 LDFLAGS	+=	-fPIC
-CFLAGS	+=	-O2 -L${LIBDIR} -I../xpdev
+CFLAGS	+=	-O2 -L${LIBDIR} -I../xpdev -shared
 ifeq ($(OS),Darwin)
  CFLAGS		+=	-D__unix__
  LDFLAGS	:=	$(CFLAGS) -dynamiclib -single_module
@@ -86,7 +86,7 @@ endif
 #
 # Link flags.
 #
-LDFLAGS	+=	-L../xpdev/$(LD).$(os).lib.$(BUILDTYPE)
+LDFLAGS	+=	-L../xpdev/$(LD).$(os).x64.lib.$(BUILDTYPE)
 #
 #------------------------------------------------------------------------------
 #