fix: #16 use CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR for…

… cmake includes

This has the benefit of working better and more reliably when the project
is included in another master project. We can guarantee that the includes
still come from the sub-project as expected.

CMakeLists.txt

@@ -15,7 +15,7 @@ cmake_minimum_required(VERSION 3.14)
 # List of directories specifying a search path for CMake modules to be loaded by
 # the the include() or find_package() commands before checking the default
 # modules that come with CMake.
-list(APPEND CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake)
+list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake)
 
 # ------------------------------------------------------------------------------
 # Project description and (meta) information