syslog-ng.pc.in: Move datarootdir higher up

For pkg-config to work correctly, we need to define datarootdir before
using it, so move it higher.

Reported-by: Viktor Tusa <tusa@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>
value-pairs: replace trash stack with own stack implementation

Because of the g_trash_stack and its usage problems could arise when
the strict-aliasing compile option was set leading to bad value-pairs
expansion. With the own implementation the bogous usage of g_trash_stack
is supposed to be eliminated.

Signed-off-by: Daniel Gados <gdani@balabit.hu>