On OS X 10.4 and newer, we have to use dlsym because the old NS* interface has
been deprecated. The patch checks for HAVE_DLFCN_H instead of switching on
the OS version.

There is one additional quirk: although OS X prefixes global symbols with an
underscore, dlsym expects its argument NOT to have a leading underscore. As a
hack, we simply strip it off in lookupSymbol. Something a bit more elaborate
might be cleaner.