doc: Add unified docs for configuration autotools/CMake/manual builds

[hebasto]

Closes the last item in the "Affecting both build systems" group in #1235.

Draft for now, as it is based on #1291.

[sipa]

I'd prefer the documentation to have a single table with all configuration options, perhaps separated into sections, but not in separate tables. It's a bit chaotic now.

[real-or-random]

Hm, I tend to think that I like the current layout, with the top-level sections being the different options (and not the build systems). It makes it easy to have larger description texts, e.g., the explanation of the window size.

But yeah, it's not very elegant when the tables all have different column widths (if this is what you dislike @sipa). In principle, it seems possible to set the column widths:

<table>
<tr><td width="500">foo</td><td>bar</td>
<table>

But that's hard to read as plain text, and it's deprecated HTML (GitHub doesn't like style="width:500px")... Not convinced.