On Mac, require at least CMake 2.6.4 for an important bug fix.

2.6.4 fixes a Mac bug in get_target_property(... "SLPlugin" LOCATION): before
that version it returns "pathname/SLPlugin", whereas the correct answer is
"pathname/SLPlugin.app/Contents/MacOS/SLPlugin". With 2.6.2, the Mac build
breaks in a mysterious way. Changing this version requirement should clarify
the solution.