configure: Do not preset $with_libstatgrab and $with_libdevstat.

Those variables are set when parsing the command line options and evaluated
inside the AC_ARG_WITH() macro. Presetting them overwrites any command line
options.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>