petscvariables: hardwired build dir instead of install dir

[Lisandro Dalcin]

Barry, I noticed that in the 'petscvariables' file, some of the config
vars for external packages (XXX_INCLUDE and XXX_LIB) have hardwired
the build dir as basepath, instead of the install dir given with
--prefix. This smells bad, but I'm not sure. What do you think? I tend
to think that every dir/file inside $PETSC_DIR should have its
basepath defined in terms of
"$PETSC_DIR/$PETSC_ARCH/innerdir[/filename.ext]".

-- 
Lisandro Dalcín
---------------
Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)
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