Scatter the useful files into subdirectories by theme.

This is maybe a little random, but better ideas seem thin on the ground.

  * `build/' is for build tools: scripts and build-tool fragments which
    are handy for building projects.

  * `doc/' is for documentation, and things which help with producing
    documentation, such as styles.

  * `licence/' contains licence documents.

  * `m4/' will be for Autoconf machinery, but it's not there yet:
    splitting up `aclocal.glob' will be a job for another commit.

  * `src/' contains source code which is shared among many projects.

This commit just moves the files about.  Some of the scripts have had
their `.in' suffix stripped because they should no longer be hacked by
Autoconf or anything similar, but none of the content is modified.