Be able to set arbitrary Xcode variables from the profile

Xcode has a lot of build settings and it doesn't make sense to wrap them all in qmake, but it still makes sense to be able to have some control over them in .pro files.

Since it is possible for users to look up the variable name in Xcode, we can support a syntax like:

xcode.SOME_VARIABLE=blah

where SOME_VARIABLE is some value from the Xcode project settings. This would do nothing on other platforms, but it would ease some of the pain of regenerating the project file when you add source files to the .pro file.