Upgrade libsvm to newer version

[jpfeuffer]

https://trac.macports.org/ticket/37862
We should upgrade. Probably related to failing SVM related tests on some of our builds.
Was looking for allocation errors in our Wrapper for hours, until I found this bug which describes my problem.

[hroest]

it seems to me we already discussed exactly this issue before: OpenMS/OpenMS#521

[hroest]

yes, I think we decided to use 3.12 -- we should probably add a comment to the contrib code to read those tickets before we change the version number

[hroest]

libsvm is now at version 3.23 and Ubuntu 18.04 packages 3.21 (see https://launchpad.net/ubuntu/bionic/+package/libsvm-dev) so we should start to look at our code and whether we can fix it for a newer version of libsvm. Debian packages 3.21 since stretch:

https://packages.debian.org/search?keywords=libsvm&searchon=names&suite=all&section=all