Xcode: Add support for combined install on iOS

This patch solves the problem of installing both: Device and Simulator
libraries on iOS. Before only one of them was installed.

If the IOS_INSTALL_COMBINED property is set on a target, a
special install hook will be activated which builds the corresponding
target and combines both at the install location.

The original patch was contributed by Ruslan Baratov, and polished by
Gregor Jasny.