Add fix for PETSc detection on Debian 9/Ubuntu 16.04
This MR fixes a problem with detecting PETSc with later Debian/PETSc packages, which is currently preventing the Debian 9 buildbot from compiling properly.
This MR fixes a problem with detecting PETSc with later Debian/PETSc packages, which is currently preventing the Debian 9 buildbot from compiling properly.