[petsc-dev] Stale lib/petsc/conf/petscvariables interferes with prefix tests

[Jed Brown]

If the most recently run configure is different from the prefix install
you want to test, then incorrect variables get resolved.  For example, I
run

  make PETSC_DIR=/home/jed/usr/petsc-mpich-basic-prefix PETSC_ARCH="" test

and lib/petsc/conf/petscvariables contains

  PETSC_ARCH=ompi-opt-singlelib                                                                                                                                                                 
  PETSC_DIR=/home/jed/petsc
  include /home/jed/petsc/ompi-opt-singlelib/lib/petsc/conf/petscvariables

which in my case defines variables like HYPRE_LIB which should be empty
in the prefix install I'm trying to test.

I think we normally don't notice because the correct petscvariables gets
included later and supersedes anything defined here.  Why isn't it spelled

  include $(PETSC_DIR)/$(PETSC_ARCH)/lib/petsc/conf/petscvariables