doc: add dep. on stamp file for rebuilding gzipped man pages.
In [1] Daniel observed that the gzipped man pages were only being
rebuild every second time when building with `make -j4'. This may be
caused by a race condition between sphinx-build rebuilding the roff
files and the recipe to gzip them. This commit sequentializes these
two steps by making the stamp file a prerequisite for (all of) the
gzip files.