Correct cmake version requirement (#1643)

* upgrade minimum cmake version supported: cmake 3.15 is first version accepting object lists for TARGET_OBJECTS

CMakeLists.txt

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: MIT
 
-cmake_minimum_required (VERSION 3.5)
+cmake_minimum_required (VERSION 3.15)
 # option() honors normal variables.
 # see: https://cmake.org/cmake/help/git-stage/policy/CMP0077.html
 if(POLICY CMP0077)