Fix handling of cpp.cxxStandardLibrary.

This fixes a bug where -stdlib=libc++ would be added unconditionally,
and makes sure not to add -stdlib=undefined to the CLI invocation.

Also, the Qt.core template has been adapted to use the new property.

Change-Id: I1dd12097edd992d851c52287e86572d6ef21bfac
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Introduce cpp.useObjcAutomaticReferenceCounting.

Very commonly toggled by Apple developers.

Change-Id: Ic17ffb0a2ba050924b8c8ce7ed14a3c8635c994d
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>