[cmake-developers] iOS support

[Eric Wing]

Thought of one more.
I hate how the top, default target is ALL_BUILD. This is problematic
for both Xcode and Visual Studio because when you use the big giant
"run" button in the UI, the IDE is confused because ALL_BUILD is an
aggregate target and not a real thing that can be run. At least for
Xcode, a bunch of options change dynamically based on the type of
target selected and building/launching to the device/simulator is not
an option when on an aggregate target.

It's an annoyance because it interferes with the normal workflow for
those familiar with the IDEs. And it's an annoyance for those who are
not familiar with the IDEs because the big giant buttons do nothing
and they don't understand the IDEs well enough on how to correct the
issue.

(It's also more work invoking xcodebuild and msbuild because you can't
rely on the default targets.)

Thanks,
Eric