On some systems a special library dir is in the cc library search, or added by -L to the libpath
but this path is missing from the loader configuration, so runtime dlopen attempts will fail.
Most prominently pcre on macports or fink, missing /opt/local/lib.
Hints may add a new key dynext_libs to add such a path to DYNEXT for loadlib.
Also provide a new PARROT_DYNEXT to manually set such paths for the runtime.
Add a helper function add_env_paths() to add multiple paths from an enviroment variable
to some library search path. Currently PARROT_INCLUDE and PARROT_LIBRARY only accept one path element.