The directory third-party/seqan is a subset of SeqAn 1.4.1 that ships with
khmer with changes they made. See third-party/seqan/ChangeLog for details. The
most important change is a bugfix which was acknowledged by the SeqAn developers
but will not be fixed until the version 2.0 release of SeqAn:
https://github.com/seqan/seqan/issues/720

Skip packaging khmer v2.1.2 until cython version 0.26.1 in in Debian or
https://github.com/cython/cython/commit/18bba8ff0261206db9243c2b1721633cb798af54
is patched in
