[CMake] a backwards compatible language simplification

[Ken Martin]

I incorporated some changes from Lloyd's patch and committed it to CVS.
Basically if a project sets CMAKE_ALLOW_LOOSE_LOOP_CONSTRUCTS on then you
can leave off the arguments for the endif endwhile, else and endforeach
commands. For example:

if (FOO AND BAR)
else ()
endif ()

The parenthesis are still required. I'm not sure what everyone's thoughts
are on this but there it is...

Ken