doxygen: add make target and use relative paths

doxygen: add make target and use relative paths

Add a make target, such that 'make doxygen' works (both for in-tree and
out-of-tree builds).  This now generates the doxygen in doc/doxygen/,
rather than in doxygen/.

While doing so, instead of genering docs with full path names (e.g.
/home/steffan/dev/openvpn/src/openvpn/crypto.h), use a relative path wrt
the project root (e.g. src/openvpn/crypto.h) in the generated
documentation.  This makes the generated doxygen easier to read.

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1510143174-15248-1-git-send-email-steffan.karger@fox-it.com>
URL: https://www.mail-archive.com/search?l=mid&q=1510143174-15248-1-git-send-email-steffan.karger@fox-it.com
Signed-off-by: Gert Doering <gert@greenie.muc.de>