bootstrap.bat fails for toolset mingw

[Michael H Kelley <mhkelley2017@…>]

See also ticket #12997

Have successfully built several versions of boost going back about 2 years. Each build fails until I appropriately tweak the bootstrap and config scripts. For 1.65.1, the fix was pretty easy, requiring only a simply manual addition to

..tools/build/src/engine/config_toolset.bat

Near the end of that script (the last toolset it checks) is mingw. Unfortunately, jam doesn't understand toolset mingw, so the toolset needs to be set to gcc. It doesn't work to call bootstrap.bat with gcc because then subsequent scripts assume either visual studior or msvc. The fix is to insert the line

set "BOOST_JAM_TOOLSET=gcc"

immediately after the line :Skip_INTEL_WIN32

then run b2 with toolset=gcc.