Use 7z if exist

7z can compress with higher ratio than gzip/zip.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57060 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/make-snapshot

@@ -44,6 +44,10 @@ PACKAGES = {
   "zip"  => %w".zip     zip -qr",
 }
 
+if system("7z", out: IO::NULL)
+  PACKAGES["gzip"] = %w".tar.gz 7z a -tgzip -mx"
+  PACKAGES["zip"]  = %w".zip    7z a -tzip  -mx"
+end
 if gzip = ENV.delete("GZIP")
   PACKAGES["gzip"].concat(gzip.shellsplit)
 end