more robust BOOST_TOOLSET name guessing from from CC, CXX

[lightmare]

There are two issues with basename $CC.

If CC is two words (e.g. ccache gcc), basename returns the first (the second word is treated as a suffix to remove, most often it doesn't match so you don't notice).
If CC is more than two words, basename complains there's too many arguments.

This PR first strips ccache if it's the first word, and then calls basename with only the first word of the rest.

[springmeyer]

Thanks @lightmare!