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]