Project: Mac: add QMAKE_MAC_SDK

I got this error while compiling for OSX
   $ ~/Qt/5.3/clang_64/bin/qmake
   Project ERROR: Could not resolve SDK path for 'macosx10.8'

And check what version of SDK do I have, I found there are 10.9 and 10.10
   $ ls /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/
   MacOSX10.10.sdk MacOSX10.9.sdk

I think we should all use the latest SDK, so I add this in project file
   QMAKE_MAC_SDK = macosx10.10

Signed-off-by: AceLan Kao <acelan@acelan.idv.tw>