Disallow in-source builds

Rationale:

1. Building in-source creates a lot of untracked files in the source
   tree. We need to take care not to commit any of them, hence we
   have to maintain .gitignore files in several directories.

2. Tests reveal that there are even files in git that are modified
   in the source tree when building docs etc. which means e.g.
   that builds can't be "repeated" after upgrading the sources, and
   that those files could be committed by accident. To avoid this
   we'd have to reorganize the build procedure and make it even more
   complex rather than simplifying it.

3. With autoconf/configure/make we didn't have a choice, i.e. we had
   to allow in-source builds with configure/Makefiles anyway. This
   is no longer true since we removed configure/make support.

Note, todo: if this commit doesn't introduce unexpected issues, then
   we can remove a lot of stuff from the .gitignore files, but this
   is left for a later commit.

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