openvas-libraries & doxygen documentation

[Lukas Schwaighofer]

Hi,

thanks for uploading openvas-libraries.  I had changed that package
quite a bit a while ago [1].  Now that it was uploaded I re-checked
what I did.  Since I learned a few new things since then, I feel bad
about causing all the buildds to all generate the doxygen documentation
even though it's only needed for the architecture independent package.

I changed debian/rules to only build the doxygen documentation in the
"override_dh_auto_build-indep" target and moved doxygen from
Build-Depends to Build-Depends-Indep.  I tested both arch-all (only)
and arch-any (only) builds using sbuild and this seems to work.
Nevertheless, please check if the changes are fine (I have pushed them
to git).

Thanks & regards
Lukas

[1] https://lists.alioth.debian.org/pipermail/pkg-security-team/Week-of-Mon-20170717/001670.html