set ASPECT_USE_PETSC for test plugins if necessary

this fixes the problems that plugins would think they have to compile
with Trilinos even though ASPECT is compiled with PETSc.
have separate plugins pick up ASPECT_USE_PETSC

Same problem as with the test plugins. Now the define ASPECT_USE_PETSC
will be set correctly if ASPECT was compiled with PETSC support.
Merge pull request #185 from tjhei/fix_petsc_plugins

Fix plugins when using PETSc