[m-rev.] diff: make it possible to use a different tar command

[Julien Fischer]

Make it possible to use a different tar command.

Mmakefile:
     Make it possible to specify a different tar command on the
     command line.  The tar supplied with OS X on my machine doesn't
     handle symlinks in the same way as GNU tar, so I need to use
     the latter (which is called gnutar).

diff --git a/Mmakefile b/Mmakefile
index 1bfa8d0..7df3fd7 100644
--- a/Mmakefile
+++ b/Mmakefile
@@ -434,6 +434,8 @@ TODO: compiler/notes/todo.html

  # Generally you should do a `mmake realclean' before doing `mmake tar'.

+TAR = tar
+
  .PHONY: tar
  tar: 	$(GENERATED_DOCS)
  	touch Mmake.params
@@ -490,7 +492,7 @@ tar: 	$(GENERATED_DOCS)
  			mv $$ROOTNAME/bytecode stuff-to-exclude; 	\
  		mv $$ROOTNAME/extras/quickcheck stuff-to-exclude;	\
  		mv $$ROOTNAME		mercury-srcdist-$(VERSION);	\
-		tar --exclude ".git" --dereference -cf - 		\
+		$(TAR) --exclude ".git" --dereference -cf - 		\
  			mercury-srcdist-$(VERSION) | 			\
  			gzip -9 > mercury-srcdist-$(VERSION).tar.gz;	\
  		mv mercury-srcdist-$(VERSION)	$$ROOTNAME;		\