chore, check for curl during configure time

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
chore, only spawn .tzmap d/l'ing and .tzmcc generation if curl is available

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'bld/tzmaps'

* bld/tzmaps:
  chore, only spawn .tzmap d/l'ing and .tzmcc generation if curl is available
  chore, check for curl during configure time