Build only the lib for subproject builds?

[jdumas]

Hi there,

When building partio as a subdirectory with CMake, it would be great if we could only build the main lib: https://github.com/wdas/partio/blob/9288fefd3b77085507171e5acdafc12cc8f44fae/CMakeLists.txt#L136 And skip the python/tools/etc. (i.e. don't require GLUT/OpenGL/Python).