python/CMakeLists.txt: Fix install destination

Added CMAKE_INSTALL_SITELIBDIR cached variable.
Default value is set to Python3_SITELIB (absolute path),
but can be changed at configuration time.

src/python/CMakeLists.txt

@@ -15,7 +15,10 @@ add_test(NAME pywraps2_test COMMAND
 set_property(TEST pywraps2_test PROPERTY ENVIRONMENT
              "PYTHONPATH=$ENV{PYTHONPATH}:${PROJECT_BINARY_DIR}/python")
 
+set(CMAKE_INSTALL_SITELIBDIR "${Python3_SITELIB}"
+    CACHE STRING "Python site-packages installation directory")
+
 # Install the wrapper.
-install(TARGETS _pywraps2 DESTINATION ${Python3_SITELIB})
+install(TARGETS _pywraps2 DESTINATION "${CMAKE_INSTALL_SITELIBDIR}")
 install(FILES "${PROJECT_BINARY_DIR}/python/pywraps2.py"
-        DESTINATION ${Python3_SITELIB})
+        DESTINATION "${CMAKE_INSTALL_SITELIBDIR}")