Change ThreadCPUUsage to call ProcessCPUUsage if __rtems__ is defined.

RTEMS real time OS doesn't support CLOCK_THREAD_CPUTIME_ID. See
https://github.com/RTEMS/rtems/blob/master/cpukit/posix/src/clockgettime.c#L58-L59

Prior to this change, ThreadCPUUsage would fail when running on RTEMS with:
  ERROR: clock_gettime(CLOCK_THREAD_CPUTIME_ID, ...) failed

* 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

Add definition of ${VAR} to makefiles if specified as part of cmake
parameter.

* Add ClearRegisteredBenchmark() function.

Since benchmarks can be registered at runtime using the RegisterBenchmark(...)
functions, it makes sense to have a ClearRegisteredBenchmarks() function too,
that can be used at runtime to clear the currently registered benchmark and
re-register an entirely new set.

This allows users to run a set of registered benchmarks, get the output using
a custom reporter, and then clear and re-register new benchmarks based on the
previous results.

This fixes issue #400, at least partially.

* Remove unused change

This reverts commit af542061.

This removes warnings when using CMake >= 3.3 if you have symbol visibility set.

* remove unnecessary weights

* use sample standard deviation

* add contributor information

* remove redundant code

* initialize variable to eliminate compiler warning

* Fix #342: DoNotOptimize causes compile errors on older GCC versions.

DoNotOptimize uses inline assembly contraints to tell
the compiler what the type of the input variable. The 'g'
operand allows the input to be any register, memory, or
immediate integer operand. However this constraint seems
to be too weak on older GCC versions, and certain inputs
will cause compile errors.

This patch changes the constraint to 'X', which is documented
as "any operand whatsoever is allowed". This appears to fix
the issues with older GCC versions.

However Clang doesn't seem to like "X", and will attempt
to put the input into a register even when it can't/shouldn't;
causing a compile error. However using "g" seems to work like
"X" with GCC, so for this reason Clang still uses "g".

* Try alternative formulation to placate GCC

Using target_include_directories CMake will implicitly add the the
necessary include paths to targets which link against the benchmark
library. This is useful when the benchmark repo is included as a
subdirectory in another CMake build.

This thing with the pragma ignore was getting out of hand: now
MinGW (and probably GCC) was erroring too. So I chose to move
the definition of IsZero() out of the anonymous namespace into
benchmark.cc.

Merge branch 'test_usercounters' of git://github.com/biojppm/benchmark into biojppm-test_usercounters

This prevents errors when additional non-timing data are present in
the JSON that is loaded, for example when complexity data has been
computed (see #379).

[ci-skip]