[release,bugfix] Fixes running the doxygen documentatio with every make call.

The cause for the problem was the introduction of cmake/modules. This caused
the find statement to always print something. This patch simply excludes
cmake/modules from the result with grep -v.
(cherry picked from commit f9ce322cde4455c790099b8ebcc47d4702c79c67)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>