* `CMakeLists.txt`: drop hopefully obsolete code
* README.md: update
* Unbreak `BENCHMARK_HAS_CXX11` macro
835365f99a stopped defining it,
but didn't un-conditionalize the code guarded under it...
* Drop `BENCHMARK_HAS_NO_VARIADIC_REGISTER_BENCHMARK`
We no longer support such an old gcc version
* `docs/user_guide.md`: proofread
* Add a test to ensure that `benchmark.h` remains C++14 header
* Revert `[[maybe_unused]]` changes - it requires C++17
* Also support C++11 standard for using the library
I don't think we want to support C++03 though,
but i suppose C++11 is palatable, at least right now.
