Fixes default hidden symbol visibility for our linux->osx cross build. Without this change, the check for working -fvisibility=hidden fails, and all symbols are visible by default.
Because the darwin linker doesn't have an option like exclude-libs=all, this visibility is actually significant. It also cleans up hundreds of lines of debug spew about conflicting visibility at link-time.
Ugly as this is, it's just a simple find/replace to fix a bug in Qt's configure. They assume in an "XPLATFORM_MAC" block that the builder is capable of running osx programs. This should be "BUILD_ON_MAC" instead.