For selecting platform compilers (and various compiler options) we set up a few variables that are passed down from the jam command line via the -s option. Our root Jamrules has some reasonable defaults set with the '?=' assignment.

For example, the command 'jam -sCORE_BUILD_COMPILER=MSVC7' will set the Jam variable 'CORE_BUILD_COMPILER' equal to the string 'MSVC7' which our scripts will detect internally and use to decide which rules to invoke. We pretty much rewrote all of the stock Jambase rules for compilation to get this to work seemlessly across multiple platforms and multiple compilers.

We have something like a dozen different knobs that can be adjusted using these -s type options. Developers tend to make scripts on their local machine to invoke Jam with the long string of -s options to create the type of build they are targeting for a particular product.