adjoints_ex6 requires a solver package.
adjoints_ex6 requires a solver package.

Fix for --disable-optional builds.

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