Re-enable the static-ruby check

After a long discussion and a lot of work in #88, we discovered that
there are two issues at play here that are out of our control.

1) Heroku runs it's own custom static-link Ruby which we can't get
working with Rice.

2) Ruby 2.2.2 and on has some serious issues with static linking in
general and there doesn't seem to be any pressure to fix this.

With that, I have to re-enable the static Ruby check so people know when
they are trying to run Rice on a platform that won't support it.

extconf.rb

@@ -6,10 +6,6 @@
 #
 # With this, installing on any of the Ruby versions installed on a machine is as
 # easy as /path/to/the/gem install rice.
-#
-# This isn't actually an extconf.rb file, all it needs to do is create a Makefile
-# for gem to continue 'make'-ing. It needs to be named 'extconf.rb' to fit Rubygem's
-# expectations
 
 $:.unshift File.expand_path(File.dirname(__FILE__))
 
@@ -22,10 +18,24 @@
 other_opts = ""
 env = ""
 
+if RbConfig::CONFIG["ENABLE_SHARED"] == "no"
+  abort <<EOC.chomp
+Unfortunately Rice does not build against a staticly linked Ruby.
+You'll need to rebuild Ruby with --enable-shared to use this library.
+
+If you're on rvm:   rvm reinstall [version] -- --enable-shared
+If you're on rbenv: CONFIGURE_OPTS="--enable-shared" rbenv install [version]
+
+If you are using Heroku, they use a own custom configured, staticly linked Ruby
+for their stack and it unfortunately does not work with Rice. You will need to use
+a custom Ruby build pack that builds and uses a non-static version of Ruby.
+EOC
+end
+
 if RUBY_PLATFORM =~ /darwin(\d+)/
   # TY Nokogiri!
   if !File.exist?('/usr/include/iconv.h')
-    abort <<'EOM'.chomp
+    abort <<EOM.chomp
 -----
 The file "/usr/include/iconv.h" is missing in your build environment,
 which means you haven't installed Xcode Command Line Tools properly.