[CMake] Better BlueGene/P and cross-compile support for CMake

[Brad King]

Todd Gamblin wrote:
> CMake cross-compile support is assuming that there is only
> one (or maybe a few) directories where target software might be
> installed.

I view this as CMAKE_FIND_ROOT_PATH helping for platforms whose SDKs
are organized that way.  If there is no root and all the target
library directories are mingled with those of other architectures
then I agree CMAKE_FIND_ROOT_PATH should not be used.  Instead the
platform file should just provide CMAKE_SYSTEM_LIBRARY_PATH and
similar variables without including UnixPaths.cmake.

-Brad