The paths to the foreign libraries have been simplified, making use of
CFFI's statement "Most, if not all, Lisps supported by CFFI have a
reasonable default search algorithm for foreign libraries."
Therefore, the paths have been eliminated. If they need to be added,
this should be done by modifying the variable
cffi:*foreign-library-directories*. The header files location set
with cc-flags for Darwin has been changed to use (gsl-config "--cflags").