/usr/lib/petscdir/3.1/conf/petscvariables is in libpetsc3.1-dev 3.1.dfsg-11ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | PETSC_ARCH=linux-gnu-c-opt
PETSC_DIR=/usr/lib/petscdir/3.1
include ${PETSC_DIR}/${PETSC_ARCH}/conf/petscvariables
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