plugins/elements/cmake.yaml: always specify variable types
Description
cmake sometimes misinterprets relative paths as relative to the current directory if the type of the variable is not specified.
See for examlpe freedesktop-sdk/freedesktop-sdk#431 (closed)
Changes proposed in this merge request:
- specify the type of CMAKE_INSTALL_LIBDIR
This merge request, when approved, will close: #761 (closed)