Fix plugins when using PETSc

This fixes the problems that plugins and test plugins would think they have to compile with Trilinos even though ASPECT is compiled with PETSc. The reason is that we never added the #define ASPECT_USE_PETSC for plugins. Luckily we don't have to change the plugin CMakeLists.txt.
This works, but don't we have access to the "real" variable ${ASPECT_USE_PETSC} in the same way as we have access to ${ASPECT_INCLUDE_DIRS} immediately above? It may be easier to read in the file you get after replacing all @STRING@ occurrences.