guile: fix build for 2.0.9

I did another pull request, but I messed up, so I closed it and created this new one, sorry for that.

Patch has been inlined and only applied if build.devel?.

This is a fix for building guile 2.0.9.

   brew install --devel guile

By error, the maintainers kept a reference to a wrong shebang shell in one of the scripts (used when building in OS X).