Restore test for $SLEPC_DIR/$PETSC_ARCH/include
Restore test for $SLEPC_DIR/$PETSC_ARCH/include

This was mistakenly deleted by #2178, which broke SLEPc configuration
for installs with a separate $PETSC_ARCH subdirectory.

capitalaslash/libmesh,hrittich/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,hrittich/libmesh,dschwen/libmesh,BalticPinguin/libmesh,libMesh/libmesh,jwpeterson/libmesh,roystgnr/libmesh,libMesh/libmesh,hrittich/libmesh,libMesh/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,hrittich/libmesh,capitalaslash/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,libMesh/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,hrittich/libmesh,libMesh/libmesh,roystgnr/libmesh,hrittich/libmesh,hrittich/libmesh,jwpeterson/libmesh,dschwen/libmesh,roystgnr/libmesh,capitalaslash/libmesh,roystgnr/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,dschwen/libmesh,dschwen/libmesh,capitalaslash/libmesh,libMesh/libmesh,dschwen/libmesh,hrittich/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,hrittich/libmesh,jwpeterson/libmesh,libMesh/libmesh,dschwen/libmesh,roystgnr/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,dschwen/libmesh,BalticPinguin/libmesh,dschwen/libmesh,libMesh/libmesh