add tarball-producing targets to Makefile for ease of release

my old, out-of-tree release script that performed a clone rather than
using git archive checked the VERSION file to make sure that it
matched before doing a release. I believe there should be a way to do
the same with git commands (without resorting to checking out the
desired tag) but I have not yet found a way, so care should be taken
when using these targets that the correctness of the VERSION file is
not overlooked.

Makefile

@@ -171,7 +171,11 @@ install-headers: $(ALL_INCLUDES:include/%=$(DESTDIR)$(includedir)/%)
 
 install-tools: $(ALL_TOOLS:tools/%=$(DESTDIR)$(bindir)/%)
 
+musl-git-%.tar.gz: .git
+	 git archive --format=tar.gz --prefix=$(patsubst %.tar.gz,%,$@)/ -o $@ $(patsubst musl-git-%.tar.gz,%,$@)
 
+musl-%.tar.gz: .git
+	 git archive --format=tar.gz --prefix=$(patsubst %.tar.gz,%,$@)/ -o $@ v$(patsubst musl-%.tar.gz,%,$@)
 
 .PRECIOUS: $(CRT_LIBS:lib/%=crt/%)