CMake: Avoid list(PREPEND ...)

list(PREPEND ...) is CMake 3.15 but our minimum is 3.13.

CMakeLists.txt

@@ -2,7 +2,7 @@ cmake_minimum_required (VERSION 3.13)
 
 project (Https CXX)
 
-list (PREPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
+set (CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake;${CMAKE_MODULE_PATH}")
 
 add_subdirectory (src)