meson: windows: Determine path to tmp_install + prefix using meson

meson: windows: Determine path to tmp_install + prefix using meson

Previously some paths (like c:\ or d:/) worked, but plenty others (like
/path/to or //computer/share/path) didn't. As we'd like to change the default
prefix to /usr/local/pgsql, that's a problem.

Instead of trying to do this in meson.build, call out to the implementation
meson install uses. This isn't pretty, but it's more reliable than what we had
before.

Discussion: https://postgr.es/CAEG8a3LGWE-gG6vuddmH91RORhi8gWs0mMB-hcTmP3_NVgM7dg@mail.gmail.com