gem: setup-osx-native-builders to set up rubies for native gem

scripts/setup-osx-native-builders

@@ -0,0 +1,50 @@
+#! /usr/bin/env bash
+
+# for inspiration / related cross-ruby building, see:
+# - https://github.com/rake-compiler/rake-compiler/blob/master/tasks/bin/cross-ruby.rake
+# - https://github.com/apolcyn/grpc/blob/master/tools/distrib/build_ruby_environment_macos.sh
+
+# chosen so chruby will see it, should you be a user of chruby
+RUBIES_DIR=${HOME}/.rubies
+
+# prerequisites on OSX
+if [[ -z "$(which ruby-install)" ]] ; then
+  echo "ERROR: ruby-install is not installed, please install it: https://github.com/postmodern/ruby-install"
+  exit 1
+fi
+
+set -o errexit
+set -o pipefail
+
+CROSS_FILE=".cross_rubies"
+RUBIES=$(cat $CROSS_FILE | fgrep darwin | cut -d. -f1,2 | sort -u)
+RAKE_COMPILER_CONFIG_DIR=${HOME}/.rake-compiler
+RAKE_COMPILER_CONFIG=${RAKE_COMPILER_CONFIG_DIR}/config.yml
+
+mkdir -p $RAKE_COMPILER_CONFIG_DIR
+echo "---" > $RAKE_COMPILER_CONFIG
+
+for ruby in $RUBIES ; do
+  ruby_fullname="native-builder-${ruby}"
+  ruby_minor="${ruby}.0"
+
+  rbconfig=$(echo ${RUBIES_DIR}/${ruby_fullname}/lib/ruby/${ruby_minor}/*/rbconfig.rb)
+  if [[ ! -e $rbconfig ]] ; then
+    echo "installing $ruby_fullname ..."
+    ruby-install -i ${RUBIES_DIR}/${ruby_fullname} ruby ${ruby} \
+                 -- \
+                 --enable-static \
+                 --disable-shared \
+                 --without-gmp \
+                 --disable-install-doc
+    rbconfig=$(echo $rbconfig)
+  else
+    echo "ruby $ruby_fullname is installed"
+  fi
+
+  platform=$(basename $(dirname $rbconfig))
+
+  echo "rbconfig-${platform}-${ruby_minor}: \"${rbconfig}\"" >> $RAKE_COMPILER_CONFIG
+done
+
+cat $RAKE_COMPILER_CONFIG