CMakeLists.txt: set minimum version to 3.7
cmake/cpu.cmake (at least) requires 3.7 due to its use of GREATER_EQUAL:
# in the file PATENTS. All contributing project authors may
# be found in the AUTHORS file in the root of the source tree.
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.7)
if(POLICY CMP0072)
cmake_policy(SET CMP0072 NEW)
