CMake: Use append instead of set to modify list

CMakeLists.txt

@@ -1,6 +1,6 @@
 # CMake 3.6 required for FindBoost to define IMPORTED libs properly on unknown Boost versions
 cmake_minimum_required(VERSION 3.6)
-set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${PROJECT_SOURCE_DIR}/CMakeModules)
+list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/CMakeModules")
 
 function(download_bundled_external remote_path lib_name prefix_var)
     set(prefix "${CMAKE_BINARY_DIR}/externals/${lib_name}")