Is your feature request related to a problem? Please describe.

Having build files sit next to source files means we can only build for one platform at a time. This makes it hard to develop while testing a few different build configurations at once, e.g. with cross compilation.

This will make it easier to develop windows functionality while not breaking unix functionality.

(Unpack phase during an official build is also wasteful, but this is minor.)

Describe the solution you'd like

Eventually the switch to Meson #3160 would get us this for free. But stuff can be done in the meantime with the configure script and makefile.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Just wait for Meson.

Additional context
Add any other context or screenshots about the feature request here.

Priorities

Add 👍 to issues you find important.