CPM_DIRECTORY always stores absolute non symlink paths (#379)

* CPM_DIRECTORY always stores absolute non symlink paths

When CPM is brought into a project via downloading it will be placed
in the build directory. When the build directory is a symlink the
CPM_DIRECTORY value will hold the symlink value. This means that
trying to run CMake via the real build directory will cause CPM
to fail since it considers the current CMAKE_CURRENT_LIST_DIR
to be different and will early exit.

This resolves the issue be always getting CMake to resolve symlinks
before comparing paths for CPM_DIRECTORY.

* Refactor logic based on PR review

cmake/CPM.cmake

@@ -30,8 +30,9 @@ cmake_minimum_required(VERSION 3.14 FATAL_ERROR)
 
 set(CURRENT_CPM_VERSION 1.0.0-development-version)
 
+get_filename_component(CPM_CURRENT_DIRECTORY "${CMAKE_CURRENT_LIST_DIR}" REALPATH)
 if(CPM_DIRECTORY)
-  if(NOT CPM_DIRECTORY STREQUAL CMAKE_CURRENT_LIST_DIR)
+  if(NOT CPM_DIRECTORY STREQUAL CPM_CURRENT_DIRECTORY)
     if(CPM_VERSION VERSION_LESS CURRENT_CPM_VERSION)
       message(
         AUTHOR_WARNING
@@ -93,7 +94,7 @@ set(CPM_VERSION
     CACHE INTERNAL ""
 )
 set(CPM_DIRECTORY
-    ${CMAKE_CURRENT_LIST_DIR}
+    ${CPM_CURRENT_DIRECTORY}
     CACHE INTERNAL ""
 )
 set(CPM_FILE