Cherry-pick content from #429 into BUILDING.md

[gdt]

As part of this, merge improvements in #429, and drop building instructions from the manual. (One only needs build instructions when i possession of sources, and a markdown file in the sources is more accessible than tex source, which hasn't been built.)

[gdt]

This is partially done, but likely some parts of #429 still useful. This ticket is now to go over the tip of the PR and consider merging parts of it, keeping in mind that the build system has changed somewhat in the meantime.