[CMake] Obtaining improved GNU make performance on Makefiles generated by cmake

[Bill Hoffman]

OK, so for a small project like PLplot, where it does a make in 1 to 2 
seconds if nothing needs to be done, this stuff really does does not 
matter that much.  Adding the phony targets may have actually made it 
take longer as make would have to parse that much more stuff.   As for 
the new way cmake does the depend stuff, I think the new approach is 
much better for larger projects like KDE/ParaView.  Basically for those 
larger projects make sometarget was almost unusable.  We create 
sometarget/fast to get around the problem, but people lost some depend 
checks.   It is basically a trade off between large and small projects. 
  If it only takes a fraction of a second longer for the small project 
but saves minutes on the larger projects, then lets save minutes.   The 
phony change is good because it fixes the make test issue. I think the 
makefiles are tuned well enough for me right now.  If someone wants to 
contribute patches that show improvement that is fine as well.   The 
cmake_force stuff is not used that much anyway.  It is only used for 
foo.i which are targets that are not part of the all build anyway.

-Bill