[build2] bpkg uninstall runs g++

[Boris Kolpackov]

MM via users <users at build2.org> writes:

> It seems bpkg uninstall runs g++
> why does it do that?

Both install and uninstall operations perform update as a pre-operation
in order to get the up-to-date view of what needs to be installed or
uninstalled. If something changes between install and uninstall (such
as the compiler or system headers get upgraded), then uninstall will,
admittedly surprisingly, recompile things before uninstalling.

Our plan to deal with this is manifest-based uninstall; see GH issue
#133[1]. In fact, we already have support for the installation
manifest[2] and just need to implement the uninstall part.

[1] https://github.com/build2/build2/issues/133
[2] https://github.com/build2/build2/blob/v0.16.0/NEWS#L288-L295