Grouping now requires cmake >= 3.8

cmake/CMakeLists.txt

 # Force minimal cmake version
-# Required 3.1 to set C++11 
-cmake_minimum_required(VERSION 3.1.0 FATAL_ERROR)
+# Required 3.8 to use source_group TREE 
+cmake_minimum_required(VERSION 3.8.0 FATAL_ERROR)
 
 file(SHA512 ${CMAKE_SOURCE_DIR}/CMakeLists.txt HASH_TOPLEVEL_CMAKEFILE)
 file(SHA512 ${CMAKE_SOURCE_DIR}/OpenFlipper/cmake/CMakeLists.txt HASH_LATEST_CMAKEFILE)