Merge topic branch t/build into master

Merge topic branch t/build into master

A medium sized series for the build system which improves the way the git
version string is stored in the executables and man pages. Subsequent patches
of the series remove some warts from the makefile: we no longer use order-only
dependencies and the .PRECIOUS target.

The merge results in a conflict against the "remove regex include" commit
67388cd4fae0. This is trivial to resolve, though.

* refs/heads/t/build:
  Doxify version functions.
  Doxify OV_EXCLUDE_STATIC_CALLBACKS #define.
  Makefile: Fix braino in tarball target.
  build: Remove superfluous dependency in Makefile.real.
  build: Remove the .PRECIOUS target.
  build: Improve clean targets.
  build: Get rid of directory order-only dependencies.
  build: Compile with -Wunused -Wall also on BSD.
  build: Revamp git versioning.
  build: Merge version.{c,h} into string.{c,h}.