CMake configuration files for OCCT

CMake provides standard mechanism for the package (OCCT in our case) to specify its configuration (location of components, build settings etc.) for dependent projects. We shall use this mechanism for OCCT to make it more usable by CMake-based software.

See https://cmake.org/Wiki/CMake/Tutorials/How_to_create_a_ProjectConfig.cmake_file for basic description and further references.

I suppose that implementation of CMake scripts of VTK can be used as example of how to apply this feature.

The project / package name of OCCT should be "OpenCASCADE".

At a minimum, file "OpenCASCADEConfig.cmake" should be created and installed. Additional files like "OpenCASCADEConfigVersion.cmake", "FindOpenCASCADE.cmake" can be created if reasonable.

The configuration file should indicate (in the order of decreasing priority):
- version of OCCT
- paths to headers, binaries, libs, resources
- used third-party libraries used (HAVE_*)
- installed modules / toolkits of OCCT
- other compiler and build options (e.g. compiler, static / dynamic build, etc.)