Misc. Build Process Changes

+ Call `make ; make install` instead of just `make install`.  This is closer to Rez 1 behaviour and works around a problem in the ExternalProjectAdd macro of CMake.

+ Ensure install paths are always made.  This includes the `REZ_LOCAL_PACKAGES_PATH` value as well as the variant subfolder.  Previously this was taken care of by CMake macros when installing `package.yaml` files etc.  Now that is no longer the case not having them made can cause build issues.