Fix building Boost.Build with a chosen toolset

[amyspark]

👋

Hi! This PR proposes to upstream a small change that we are shipping in Krita starting with 5.1 (https://invent.kde.org/graphics/krita/-/commit/2876937bf2dfea45645c31ebb749b1a0d08d067b).

In short, this will enable users of the CMD bootstrap file to build Boost with a toolset other than MSVC. We use it in particular to target the llvm-mingw compiler, through a custom user-config.jam. (It also applies to the official MSYS2 CLANG* flavours, which I use for my own personal development needs.)

cc @dimula73

[mclow]

This looks very similar to #682

[amyspark]

That's correct @mclow. However, the other PR neither is documented nor does it specify the rationale for the proposed change, so that's why I didn't spot it before submitting this one.

[amyspark]

@mclow I see there's a window open for bugfixes, would it be possible for this to get reviewed?

[swatanabe]

#513 looks like another duplicate

[amyspark]

Ok, closing this one in favour of #513.