Remove MSVC logic from the configure script

Remove MSVC logic from the configure script

Cargo will soon support testing the cross compilation capabilities between 32/64
bit Windows, meaning that there's not "one true value" for any of these
environment variables that Cargo is setting (e.g. where to find `cl`, `link`,
etc). Instead, all dependencies have been updated to probe the system (in the
same manner as the compiler) for the tools that they're using.

All of the logic is housed in the `gcc` crate which now exposes a function to
probe the system for a particular tool. The updated crates here then use the
result of this probe to run the various build scripts. This all boils down to
being able to build MSVC targets inside an MSYS shell instead of requiring use
of a MSVC shell (which doesn't allow for simultaneous host builds and cross
builds).