CMake: Install to /usr if CMAKE_INSTALL_PREFIX not specified

CMake/CMakeLists_root.txt

@@ -92,7 +92,10 @@ endif()
 set(CMAKE_CXX_STANDARD {{cpp_version}})
 # TODO set CMAKE_INSTALL_PREFIX in release only
-set(CMAKE_INSTALL_PREFIX /usr)
+if (CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
+    set (CMAKE_INSTALL_PREFIX /usr)
+endif()
+message ( "[Install] Install directory prefix is ${{CMAKE_INSTALL_PREFIX}}" )
 set(CMAKE_INSTALL_RPATH "${{CMAKE_INSTALL_PREFIX}}/lib")
 set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)