Factor is a new language that is gaining momentum in the open source community. A few of the core members were in town visiting after the creator Slava gave a Google Tech Talk. Over beers and pizza at the Lucky Lab NW the Factor guys told me how FFI works and we had a binding to a Bessel function working within half an hour. It's that frickin' easy!
Here is the gitweb for factor-gsl, which currently has support for all Bessel functions.
To clone master repo:
git clone http://leto.net/code/factor.git
( scratchpad ) USE: gsl.sf.bessel
( scratchpad ) 10 gsl_sf_bessel_J0 .
There are tests also, but the docs are still a bit rough.