Add user override mechanism for CFFI foreign libraries

[dieggsy]

The work so far on the CFFI port depends on cffi:define-foreign-library and cffi:load-foreign-library. We attempt to provide defaults that "should" work in the foreign library definition, but it's possible some users may need or want a more custom setup.

The variable clsql:*foreign-library-search-paths* is insufficient for this, because it is searched as a backup to default system locations, which may already contain an unwanted version of a foreign library.

On Linux, this seems trivially solved by LD_LIBRARY_PATH.

On macOS DYLD_LIBRARY_PATH is inherited by child processes by default, which poses an issue for:

• Any lisp called as a sub-process
• At least homebrew's distribution of SBCL since it's distributed with a shell script wrapper.

It seems possible to define, for example, clsql:*libodbc-library*, which would be NIL by default, but could be bound by a user to a pathname or their own CFFI foreign library definition. If this variable were bound, we would try loading this instead of the defaults.

[dieggsy]

In addition to overriding the path, a used may want to override the specific library used (e.g. prefer libiodbc even if unixodbc is installed), which is not easy on either platform currently.