Ensure 'make dist' compiles git-archive.exe on Cygwin

On Cygwin we have to use git-archive.exe as the target, otherwise
running 'make dist' does not compile git-archive in the current
directory.  That may cause 'make dist' to fail on a clean source
tree that has never been built before.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1087,7 +1087,7 @@ git.spec: git.spec.in
 	mv $@+ $@
 
 GIT_TARNAME=git-$(GIT_VERSION)
-dist: git.spec git-archive configure
+dist: git.spec git-archive$(X) configure
 	./git-archive --format=tar \
 		--prefix=$(GIT_TARNAME)/ HEAD^{tree} > $(GIT_TARNAME).tar
 	@mkdir -p $(GIT_TARNAME)