Fix build with boost >= 1.40

With boost 1.55.0-4 on arch linux, I get an error:

    /usr/bin/ld: cannot find -lboost_regex-mt

I found out that this is due to the removal of all the boost shared objects with the mt suffix, which seems to have happened around boost 1.40 because there was no more need for a seperate multithread-version of the libraries. Eventually there are still symlinks on many systems which make the usage of -lboost_(...)-mt still be valid.

I couldn't find any of the files with the suffix in the files list of the libboost_regex versions shipped in the oldest supported debian and ubuntu versions so it should probably be safe to assume every system the tntnet-homepage will be built on will have a boost version with a boost_regex library without suffix that has multithread capabilities.