GUI applications cannot be built on OS X < 10.7

This is due to the fact that plutil does not support the json plist format until OS X 10.7. We use this heavily in composition of Info.plist and PkgInfo.

The best course of action is probably to write our own version of plutil (qplutil?) which supports reading and writing all four plist formats and is cross platform. The C++ API which would power this program would also be useful in other areas... such as the Xcode project generator.

As a temporary workaround, users can build applications on < OS X 10.7 by:

• setting createPkgInfo to false (see: https://codereview.qt-project.org/60846)
• setting processInfoPlist to false
• avoiding use of infoPlistFile