Add CMake Option to Disable Static Libs

[danrbailey]

In some cases, building only the shared libraries is desirable. The default behavior remains to build both shared and static libraries.

[davidgyu]

Filed as internal issue #OSD-466

[davidgyu]

@danrbailey could you make sure to submit an OpenSubdiv Contributor License Agreement (CLA). Thanks!

[davidgyu]

Hey @danrbailey checking again to see if you could submit an individual or corporate CLA to allow us to consider merging these PRs. Thanks! -David

[nvmkuruc]

I wanted to ask about this approach. BUILD_SHARED_LIBS is a first class documented cmake variable (https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html) while BUILD_STATIC_LIBS is not.

To support the request of being able to build static without shared, would it be better to follow the guidance in BUILD_SHARED_LIBS and produce either shared or static libraries and never both?

This would align to standard cmake conventions and let the same target identifier be used in both static and shared builds.