libical: use system time zone info

libical: use system time zone info

libical requires time zone information in order to make use of time
zone functions (otherwise, segfaults occur when applications call
the time zone functions). This patch:
1. forces the system to build time zone info
2. ensures libical does not build its own time zone info (this is
   the default behaviour, but the flag is added in case libical's
   default changes in the future).

Signed-off-by: Marc Khouri <marc@khouri.ca>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>