Support standard CMake/GNUInstallDirs variables and traditional Linux install tree

`pd.cmake` currently does not support standard CMake / `GNUInstallDirs` variables, including:

- `CMAKE_INSTALL_PREFIX`
- `CMAKE_INSTALL_LIBDIR`
- `CMAKE_INSTALL_INCLUDEDIR`
- `CMAKE_INSTALL_DATADIR`
- `CMAKE_INSTALL_DOCDIR`

For additional context, see the discussion in **#22**.
