test_cxx11 in build.sh doesn't use CXXFLAGS

The `error_exit` function in `src/engine/build.sh` says:

> For any toolset you can override the path to the compiler with the CXX
> environment variable. You can also use additional flags for the compiler
> with the CXXFLAGS environment variable.

But `CXXFLAGS` is not used by the `test_cxx11` function. If the compiler you're trying to use to build B2 needs additional flags you have to pass them as part of the `CXX` env var instead, e.g. `CXX="/path/to/g++ -pthread" ./build.sh gcc`

Also, redirecting all output and errors to `/dev/null` in `test_cxx11` makes it awkward to see *why* the test fails. You need to modify the `build.sh` script to find out why it's failing and what you might need to do to solve the problem.