Base: assume and use C++11 in libbase

Assume C++11 support. logging.h already used constexpr before,
macros.h now also does no longer check the language level.

Use constexpr for the logging evaluator. This allows the usage
of CHECK and co in other constexpr expressions in C++14.

Test: m checkbuild (N9)
Change-Id: Ifdffa074271fff1f9949c48829a185800ec5e524