- the meson build assumed meson/ninja/win_flex/win_bison/perl on PATH,
which only held on machines that had them installed
- add meson, ninja and winflexbison tool packages and declare them via
tools@windows on postgresql, together with the existing strawberry-perl
- meson no longer ships a Windows MSI, so the wheel is installed offline
into a venv: postgres re-invokes meson through find_program, which
needs the real meson.exe launcher pip generates
