cmake subdirectory support

[jwinarske]

Is your feature request related to a problem? Please describe.

When using the filament folder/project from an upper level CMake project, it looks for launch-c and launch-cxx in the root build folder.

add_subdirectory(filament)

Describe the solution you'd like
Path in CMake selecting the launch scripts needs adjusting. Workaround is to copy scripts from binary subfolder to binary root, and mark for execution.

Describe alternatives you've considered
NA

OS and backend
Agnostic

[pixelflinger]

Are you able to put a PR up?

[jwinarske]

@pixelflinger sure. I'll work on that today.