Make sure yahttp is linked into remotebackend

Make sure that the yahttp directory is built first
and then link in the resulting library.

Checked with readelf -s .libs/libremotebackend.so | grep NOTYPE
that this fixes the undefined symbol.

Fixes #1732
Merge pull request #1739 from rubenk/link-yahttp-into-remotebackend

Make sure yahttp is linked into remotebackend