Use CMAKE_CURRENT_LIST_DIR instead of CMAKE_CURRENT_SOURCE_DIR in CMakeLists.txt

[htfy96]

Currently, this project uses:

project(Vc VERSION "${CMAKE_MATCH_1}.${CMAKE_MATCH_2}.${CMAKE_MATCH_3}" LANGUAGES C CXX)
list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")

to add the module directory into CMAKE_MODULE_PATH. However, it doesn't behave as expected when users try to embed this project into their own using CPM.cmake:

CMake Error at /home/lz/.cache/CPM/vc/c43a073d69c63625768cfeb76b77ea806113471c/CMakeLists.txt:25 (include):
  include could not find requested file:

    VcMacros


CMake Error at /home/lz/.cache/CPM/vc/c43a073d69c63625768cfeb76b77ea806113471c/CMakeLists.txt:26 (include):
  include could not find requested file:

    AddTargetProperty


CMake Error at /home/lz/.cache/CPM/vc/c43a073d69c63625768cfeb76b77ea806113471c/CMakeLists.txt:27 (include):
  include could not find requested file:

    OptimizeForArchitecture


CMake Error at /home/lz/.cache/CPM/vc/c43a073d69c63625768cfeb76b77ea806113471c/CMakeLists.txt:29 (vc_determine_compiler):
  Unknown CMake command "vc_determine_compiler"

This is because CMAKE_CURRENT_SOURCE_DIR only refers to the parent project's source dir when this project is embedded, which can be fixed with CMAKE_CURRENT_LIST_DIR which points to the correct directory