Avoid full-source-dist failure if no .tar.xz file is present under deps/

Only LLVM 3.5 comes as a .tar.xz, which makes this target fail when
LLVM_VER=3.3 (the default).

Makefile

@@ -444,7 +444,7 @@ full-source-dist source-dist: git-submodules light-source-dist.tmp
 
 	# Create file full-source-dist.tmp to hold all the filenames that go into the tarball
 	cp light-source-dist.tmp full-source-dist.tmp
-	ls deps/*.tar.gz deps/*.tar.bz2 deps/*.tar.xz deps/*.tgz deps/*.zip >> full-source-dist.tmp
+	-ls deps/*.tar.gz deps/*.tar.bz2 deps/*.tar.xz deps/*.tgz deps/*.zip >> full-source-dist.tmp
 	git submodule --quiet foreach 'git ls-files | sed "s&^&$$path/&"' >> full-source-dist.tmp
 
 	# Prefix everything with the current directory name (usually "julia"), then create tarball