We currently only search -mt and the empty string for boost suffixes when building with --use-system-boost.
This may not be sufficient on some systems where a user has boost installed to a non-system-searched path and has configured boost to not add the "-mt" adornment to the libraries, despite their having thread support.
When this setup is used, and we search for boost_system, for example, we will first look for boost_system-mt. The users search path (presumably configured with --extrapath or similar) will be examined for libboost_system-mt, which will not be found. We will then search the system boost installation, which does offer it. So we will choose the -mt adorned version of boost_system, and link against the library in the default search path, rather than in the users custom boost installation.
We should make it possible to specify the list of suffixes to search (including the empty suffix), as well as suppress all searches, which may be unnecessary on some platforms, like Windows, with autolib linking.