How to change the default toolset during build?

@grafikrobot

The build fails:
```

warning: No toolsets are configured.
warning: Configuring default toolset "gcc".
warning: If the default is wrong, your build may not work correctly.
warning: Use the "toolset=xxxxx" option to override our guess.
warning: For more configuration options, please consult
warning: http://boost.org/boost-build2/doc/html/bbv2/advanced/configuration.html
/usr/ports/devel/boost_build/work/build-4.3.0/src/tools/gcc.jam:226: in gcc.init from module gcc
error: toolset gcc initialization:
error: no command provided, default command 'g++' not found
error: initialized from
```

http://boost.org/boost-build2/doc/html/bbv2/advanced/configuration.html mentioned above is a broken link.

Start with the build warning and the linked advanced configuration documentation. Verify the referenced URL, then clarify how the default toolset can be changed using the documented toolset option; the work is done when the guidance is reachable and answers this build failure.

Written by the indexing model from the issue text.