FIX: builds/ should be the default output directory for rsc.r

The earlier relative path handling fix introduced a regression by always
building to the current directory, even if no explicit output path was
given. However, Red/System should build to its local builds/ directory,
unless told otherwise. So this fix now only uses the working
directory-relative path logic when given an explicit output path as
command-line option.
Merge pull request #470 from be-red/master

Red/System zlib binding
TESTS: Added tests re issue #465
TESTS: test re issue #468
TESTS: added tests re issue #420
TESTS: added tests re issue #460
TESTS: added test for issue #443
TESTS: added tests re issue #455