Add --with-thread-model=openmp option.
Add --with-thread-model=openmp option.

This PR also changes the default (when no thread model is requested)
to pthreads instead of TBB.  TBB can still be explicitly enabled by
configuring with --with-thread-model=tbb.

Since the "pthreads" model can actually use OpenMP pragmas, we
configure for OpenMP and pthreads at the same time.

libMesh/libmesh,hrittich/libmesh,dschwen/libmesh,hrittich/libmesh,roystgnr/libmesh,pbauman/libmesh,pbauman/libmesh,giorgiobornia/libmesh,BalticPinguin/libmesh,90jrong/libmesh,roystgnr/libmesh,giorgiobornia/libmesh,jwpeterson/libmesh,giorgiobornia/libmesh,hrittich/libmesh,pbauman/libmesh,BalticPinguin/libmesh,dschwen/libmesh,dschwen/libmesh,BalticPinguin/libmesh,hrittich/libmesh,giorgiobornia/libmesh,libMesh/libmesh,hrittich/libmesh,capitalaslash/libmesh,roystgnr/libmesh,libMesh/libmesh,capitalaslash/libmesh,roystgnr/libmesh,giorgiobornia/libmesh,giorgiobornia/libmesh,libMesh/libmesh,giorgiobornia/libmesh,90jrong/libmesh,dschwen/libmesh,roystgnr/libmesh,90jrong/libmesh,jwpeterson/libmesh,dschwen/libmesh,giorgiobornia/libmesh,roystgnr/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,pbauman/libmesh,jwpeterson/libmesh,hrittich/libmesh,dschwen/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,roystgnr/libmesh,giorgiobornia/libmesh,jwpeterson/libmesh,90jrong/libmesh,dschwen/libmesh,pbauman/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,libMesh/libmesh,libMesh/libmesh,dschwen/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,pbauman/libmesh,pbauman/libmesh,90jrong/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,90jrong/libmesh,BalticPinguin/libmesh,hrittich/libmesh,90jrong/libmesh,capitalaslash/libmesh,pbauman/libmesh,hrittich/libmesh,jwpeterson/libmesh,90jrong/libmesh,hrittich/libmesh,libMesh/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,pbauman/libmesh,libMesh/libmesh,90jrong/libmesh