PROTON-762: Fix Javascript bindings build
- Strangely(?!) this makes it the same as all the other binding builds.
PROTON-767: Allow send-async to compile as C89
PROTON-752: Release the GIL on blocking calls in Ruby.

Added a wrapper call for both pn_messenger_recv and pn_messenger_send that
calls the appropriate method for the version of Ruby to release the GIL
until the call returns in C.

Added to both CMakeLists.txt for the Ruby bindings, and also to the
extconf.rb file for the gem, a macro named RUBY## (where ## is 18, 19,
20 or 21 depending on the version of Ruby) to ensure the right API is
compiled into the extension since the API is different between 1.9 and
2.x and non-existent in 1.8, where it's a no-op.