petsc.m4: don't test against uninitialized variable.
petsc.m4: don't test against uninitialized variable.

Also, it appears that petscconf.h can be located in
$PETSC_DIR/$PETSC_ARCH/include for some PETSc builds, so handle that
situation as well.

Thanks as well to @roystgnr for the 1-liner that simplified the logic.

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