[CMake] Ninja generator is Linux-only?

[Bill Hoffman]

Ninja is currently not totally working on mac and windows.

The limitations are:

Windows:  file level dependencies are not working.
Mac: Frameworks and application bundles are not working.

Here are the dashboards with the failing tests:

http://open.cdash.org/viewTest.php?onlyfailed&buildid=2281938
http://open.cdash.org/viewTest.php?onlyfailed&buildid=2280676

The Mac fix should be pretty simple.  It involves moving some code out 
of the makefile generator into a shared place with the ninja generator 
so that it can create the frameworks and bundles.

The windows fix requires a change in upstream ninja.   Right now the 
ninaj cmake dashboard is using a fork of ninja to get response files. 
There is some version of that in the upstream ninja.  There is some 
discussion of some work that adds the depend information to ninja, but I 
don't think it is quite there.

Both of these issues are serious enough that the support for ninja on 
these platforms is broken as far as cmake is concerned.

-Bill