This increases the minimum necessary version of CMake to 3.1.
There are three levels:
1. TDE_CXX_FEATURES common for all TDE modules
2. PROJECT_CXX_FEATURES common at invidual module level
3. CXX_FEATURES and CXX_FEATURES_PRIVATE for individual libraries and binaries
Public CXX_FEATURES for libraries become part of the exported CMake target.
Signed-off-by: Slávek Banko <email@example.com>