biology/segan: Limit GCC to 4.8 while waiting for update

Due to questionable c++, GCC 4.9 and newer cannot build seqan. An update
has been in the works that will enable clang compilers and all versions
of GCC to build seqan, but it is not quite ready yet. While waiting for
it, limit the version of GCC used to 4.8.

PR: 196851