configure.ac: Do s/-pthreads -lpthreads/-pthreads/ for protobuf

protobuf uses an obsolete automake pthreads detection macro,
which results in "pkgconfig --libs protobuf" returning
"-lprotobuf -pthread -lpthread" on Linux.  Remove
the unnecessary and dangerous -lpthread in that case.

Fixes #727, mosh-server crash in libutempter on Debian Sid.