syslog-ng.pc.in: Add scldir and datarootdir

The former is primarily for syslog-ng-incubator, the latter is to
silence config.status and for correctness' sake.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #100 from algernon/f/scl/new-variables

syslog-ng.pc.in: Add scldir and datarootdir