The internal GL API Loader implementation is self contained and does
not require the system OpenGL implementation at build time, so we
don't need the OpenGL::GL import target for this case.
An earlier change added the OpenGL::GL import target for legacy uses
of GLEW and it remains correct there.
Also fixed the build so that obsolete CUDA gpu architectures
are specified as fallback values only when using older obsolete
versions of CUDA. We'll likely remove these fallbacks from future
releases of OpenSubdiv.
Updated target object dependencies for Apple static and dynamic frameworks
to use TARGET_EXISTS for osd_gpu_obj to allow correct linking when no
OSD_GPU targets are enabled. We'll likely revisit these targets further
for future releases of OpenSubdiv.