Unbreak compose-elemstyles.pl

The cause it didn't work was incompatibility with BSD make
Merge branch 'master' of github.com:AMDmi3/osm2go
Fix iso 8601 time handling

This must not depend on local timezone in any way, so use timegm
instead of mktime and take parsed timezone into account so both
e.g.  2013-08-02T02:09:39Z and 2013-08-02T06:09:39+0400 are correctly
parsed as 1375409379.