libxc: remove usage of "daylight" variable

libxc: remove usage of "daylight" variable

FreeBSD doesn't implement the XSI extension that mandates the presence
of the daylight variable as described in:

http://pubs.opengroup.org/onlinepubs/009696799/functions/tzset.html

So avoid using it for portability reasons. Use tm_isdst instead to
decide if daylight savings time conversions should be used or not.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>