[GH #902] Support multiple paths in PARROT_LIBRARY, PARROT_INCLUDE

Fixed conflict:
	t/library/lib_search_path.t
[GH #903] Rename dynext_libs to dynext_dirs

For consistency with the other config *dir entries.
Note that nqp also has now a --dynext-paths option.
https://github.com/perl6/nqp/pull/69