* Make Benchmark a single header library (but not header-only)
This patch refactors benchmark into a single header, to allow
for slightly easier usage.
The initial reason for the header split was to keep C++ library
components from being included by benchmark_api.h, making that
part of the library STL agnostic. However this has since changed
and there seems to be little reason to separate the reporters from
the rest of the library.
* Fix internal_macros.h
* Remove more references to macros.h
In non-debug builds CMake automatically adds -DNDEBUG, this means
that uses of `assert` in the tests are disabled for non-debug builds.
Obviously we want these tests to run, regardless of configuration.
This patch strips -DNDEBUG during non-debug builds and adds
-UNDEBUG just to be sure.
