Update CMakeLists.txt

Use `PROJECT_SOURCE_DIR` to allow embedding within larger projects. `CMAKE_SOURCE_DIR` is the root of the source tree, where as `PROJECT_SOURCE_DIR` is the directory of the current project.

Author: compnerd

CMakeLists.txt

@@ -11,7 +11,7 @@ cmake_minimum_required(VERSION 3.16)
 project(swift-numerics
   LANGUAGES Swift)
 
-list(APPEND CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake/modules)
+list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/modules)
 
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)