Check extensions were included instead of adding them with extconf.rb.

Using extconf requires an existing ruby & mkmf of a particular
version (e.g. 1.8/1.9 mkmf can't configure a 2.1 build). Instead,
just make sure the extensions we want were included on their own.
Handle versions with and without patchlevel in 'ruby.src'.

Rubies since 2.1 don't include the patchlevel in their version name.
Make sure the patchlevel begins with 'p' in 'ruby.src'.
Default to a newer ruby, 2.1.2 (which doesn't include a patchlevel).
Remove unused 'ruby trunk.src'.