OSX: Correctly set min version for Vidalia
OSX: Correctly set min version for Vidalia

Each line in a make rule gets executed in its own shell, so exporting
something in one line means it'll be gone in the next. We only need the
MACOSX_DEPLOYMENT_TARGET during cmake, so only set it there.

Fixes bug 4263.

Shondoit/torbrowser,Shondoit/torbrowser,Shondoit/torbrowser,Shondoit/torbrowser