add -compile-examples configure option

this overrides the magic that makes examples only install their sources
in production builds.

packagers may want to force the build of the examples, so they can
package them up for demo purposes.
this is actually why we formerly had the split between demos and
examples ...

Task-number: QTBUG-30788
Change-Id: I5633f69404c5aa6846f5496e8f161a273a7a7da3
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>