Fix Rubinius gem binstubs path

Rubinius 2 insists that it installs RubyGems binstubs into
`PREFIX/gems/bin` instead of `PREFIX/bin`.

This creates complexity for rbenv rehash and exec processes, so we
symlink `gems/bin` into `bin` and have RubyGems create binstubs at a
location that is consistent with other Ruby implementations.

See sstephenson/rbenv#178, sstephenson/rbenv#461

bin/ruby-build

@@ -431,6 +431,7 @@ build_package_rbx() {
   { bundle
     ./configure --prefix="$PREFIX_PATH" $RUBY_CONFIGURE_OPTS
     rake install
+    fix_rbx_gem_binstubs "$PREFIX_PATH"
   } >&4 2>&1
 }
 
@@ -516,6 +517,20 @@ fix_directory_permissions() {
   find "$PREFIX_PATH" -type d \( -perm -020 -o -perm -002 \) -exec chmod go-w {} \;
 }
 
+fix_rbx_gem_binstubs() {
+  local prefix="$1"
+  local gemdir="${prefix}/gems/bin"
+  local bindir="${prefix}/bin"
+  # Symlink Rubinius' `gems/bin/` into `bin/`
+  if [ -d "$gemdir" ]; then
+    for file in "$gemdir"/*; do
+      [ -x "$file" ] && mv "$file" "$bindir"
+    done
+    rm -rf "$gemdir"
+    ln -s ../bin "$gemdir"
+  fi
+}
+
 require_gcc() {
   local gcc="$(locate_gcc || true)"