This adds a patch to guile-fibers to fix a resource leak that caused
file descriptors to be opened and never closed with each invocation of
`run-fibers'.  This is presumably what was causing the tests to fail, as
guile will abort when it gets EMFILE while attempting to create a new
thread.  I've verified that it builds on my system, but it's only a
4-core machine, and the rate at which file descriptors leak scales with
the number of cores, so it's possible it would have built successfully
here regardless.  Could someone with access to a system with more cores
verify that it now builds properly there?

Hopefully a bug fix release will show up soon enough and we can get rid
of this.

- reepca