Add a script to build all gems

This makes it easy to build all gems locally by running
`scripts/build-gems`.

scripts/build-gems

@@ -0,0 +1,30 @@
+#! /usr/bin/env bash
+#
+#  script to build gems for all relevant platforms
+#
+set -o errexit
+set -o nounset
+set -x
+
+rm -rf tmp pkg gems
+mkdir -p gems
+
+# prelude: let's check that things work
+bundle update
+
+bundle exec rake clean clobber
+bundle exec rake compile
+bundle exec rake spec
+
+# MRI et al (standard gem)
+bundle exec rake clean clobber
+bundle exec rake gem
+cp -v pkg/re2*.gem gems
+
+# precompiled native gems ("fat binary")
+bundle exec rake gem:native
+cp -v pkg/re2*.gem gems
+
+pushd gems
+  ls *.gem | sort | xargs sha256sum
+popd