Use obj targets for faster CPU/GPU lib builds

Further leverage cmake object libraries to share object files for CPU
and GPU OSD libraries, avoiding duplicate complation for dynamic/static
build passes.

CMake restricts object library inputs to header and source files, so the
.inc files were renamed to .gen.h (which seems like a better name
anyway) to make CMake happy.

Also updated the .gitignore file to ignore .gen.h files.

Conflicts:
	opensubdiv/osd/CMakeLists.txt