For OS X, don't rigidly tie the SDK version to the minimum target version.

According to

	https://developer.apple.com/library/mac/documentation/DeveloperTools/Conceptual/cross_development/Configuring/configuring.html#//apple_ref/doc/uid/10000163i-CH1-SW1

the deployment target (minimum target OS version) and SDK version aren't
necessarily the same and, in fact, Apple typically only ship two SDKs
with each Xcode release, so if you want to build for 10.6 with the 10.6
SDK, you have to use a version of Xcode sufficiently old to have the
10.6 SDK.

Here, we instead search for the oldest SDK for an OS whose version is
greater than or equal to the deployment target.  Note that this may not
work for X11-based Wireshark, as the X11 libraries can change
incompatibly between releases.  (Fortunately, our plan is to kick
X11-based Wireshark to the curb for OS X, removing a large pile of
aggravation for users.)

This also requires some fixes when building Qt and gdk-pixbuf, as some
cases where we were using the minimum OS target version we needed to be
using the SDK version.

For CMake, we're using its native "deployment target" support for OS X,
and hope that it will somehow do the right thing.

Change-Id: Ie8f42c5e4719e7ebdc56b9ba5a330665bee06280
Reviewed-on: https://code.wireshark.org/review/5031
Reviewed-by: Guy Harris <guy@alum.mit.edu>