correctly handle CMAKE_INSTALL_PREFIX

Author: kkloberdanz

CMakeLists.txt

@@ -131,7 +131,10 @@ endif()
 
 
 option(USE_DEFAULT_INSTALL_PATH "Override the CMake default installation path" OFF)
-if(NOT USE_DEFAULT_INSTALL_PATH)
+if(NOT CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT AND USE_DEFAULT_INSTALL_PATH)
+    message(FATAL_ERROR "You cannot set both CMAKE_INSTALL_PREFIX and USE_DEFAULT_INSTALL_PATH")
+endif()
+if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT AND NOT USE_DEFAULT_INSTALL_PATH)
     message(WARNING "the CMake default install path is being overriden. This behavior will not be the default in a future release. Build with -DUSE_DEFAULT_INSTALL_PATH=ON to opt into what will be the default behavior in a future release")
     set (CMAKE_INSTALL_PREFIX "${CMAKE_BINARY_DIR}/install" CACHE PATH "default install path" FORCE)
 endif()