configure.ac: Improved the linux capability check

The Linux capabilities check was dependent only on the existence of
<sys/capability.h>. While there was a check performed for -lcap and its
usability, if that check failed, linux capability support was never
turned off.

This caused problems on recent FreeBSDs, where the header exists, but -
naturally - the library does not. This patch fixes #175, by modifying
the check to turn off the feature if the library is not available.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
configure.ac: Fix --enable-linux-caps

While fixing the auto-detection of Linux capabilities,
--enable-linux-caps got broken. This patch corrects that, so that both
work now.

Reported-by: Peter Czanik <czanik@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>