If list of valid target architectures is empty for given SDK then there will
be no VALID_ARCHS build setting returned by Xcode. Return "" (empty string)
explicitly in this case. This may happens if CMAKE_IOS_INSTALL_COMBINED is ON
but only one architecture used in target.
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.
