jit.build-all now accepts multiple arguments
jit.cc and jit.flex: no longer compile as POSIX_SOURCE

This breaks too many things on OSX.
Fixes preprocessing: jit removed one line too many
Add mruby support.
Performance improvements

By reducing the number of calls to external programs up-to-date
scripts start faster.
Reviewed build selection for running.

With this patch a distribution build is built only via jit.build(-all).

Also, the local build gets checked first, and for performance reasons
the distribution build will be copied over to the local build if the
latter does not exist.