Fix plpython build on older versions of OS X.

Pre-Lion versions of Apple's linker don't allow space between -F and its
argument.  (Snow Leopard is nice enough to tell you that in so many words,
but older versions just fail with very obscure link errors, as seen on
buildfarm member locust for instance.)  Oversight in commit
fc8745070a53469a43ecbf999dc5692a36a649cc.