Having access to the thread count from within a benchmark is useful,
for when one wants to distribute a workload dynamically among the
benchmarks running in parallel e.g when using ThreadRange() or
ThreadPerCpu().
This patch adds a `coverage` target that allows coverage statisitics to be
retrieved for the project. It requires that lcov and gcov is installed and
that the generator is unix makefiles but this can be improved upon in
future releases.
To make it work use the coverage build type:
```
cmake -DCMAKE_BUILD_TYPE=Coverage .
make coverage
```
This patch adopts a new internal structure for how timings are performed.
Currently every iteration of a benchmark checks to see if it has been running
for an appropriate amount of time. Checking the clock introduces noise into
the timings and this can cause inconsistent output from each benchmark.
Now every iteration of a benchmark only checks an iteration count to see if it
should stop running. The iteration count is determined before hand by testing
the benchmark on a series of increasing iteration counts until a suitable count
is found. This increases the amount of time it takes to run the actual benchmarks
but it also greatly increases the accuracy of the results.
This patch introduces some breaking changes. The notable breaking changes are:
1. Benchmarks run on multiple threads no generate a report per thread. Instead
only a single report is generated.
2. ::benchmark::UseRealTime() was removed and replaced with State::UseRealTime().
Fixes#89
UseRealTime was defined in the internal namespace by mistake.
Similarly, documentation suggested that benchmark::SetLabel should be
used to set a label, and a function was declared but not defined, while
actually the call should be 'state.SetLabel'.
