MSYS2 and similar MinGW/MSYS distributions define development
environments with a `MSYSTEM` environment variable.  Each such
environment has a documented installation prefix for its packages,
often provided by a `MSYSTEM_PREFIX` environment variable.
Since commit 84a25fc2 (cmake_host_system_information: Add
MSYSTEM_PREFIX query, 2023-09-08) we can look up this prefix.

Add `$MSYSTEM_PREFIX/local` and `$MSYSTEM_PREFIX` to our system search
prefixes when targeting MinGW under `MSYSTEM` environments.  This is
their equivalent to `/usr/local` and `/usr`, which we search by default
on UNIX systems.

Issue: #24216