OS X: Search for SDK based on deployment target (#14324)

When available, use CMAKE_OSX_DEPLOYMENT_TARGET instead of the host OS X
version to select the default SDK.  This makes sense because one should
use the SDK matching the deployment target.

Suggested-by: John Ralls <jralls@ceridwen.us>
bootstrap: Do not suppress CMAKE_OSX_SYSROOT if CFLAGS have -isysroot (#14324)

Revert commit a1c032b9 (bootstrap: Suppress CMAKE_OSX_SYSROOT if CFLAGS
have -isysroot, 2012-09-21).  If MACOSX_DEPLOYMENT_TARGET is set then
CMAKE_OSX_DEPLOYMENT_TARGET will be set and Darwin.cmake will complain
if no CMAKE_OSX_SYSROOT is set.  Just allow both -isysroot flags to
appear.  The one generated by CMAKE_OSX_SYSROOT appears after and
overrides the one from CFLAGS/CXXFLAGS.
Merge topic 'osx-find-sdk-for-deployment-target'

bf5a5bc bootstrap: Do not suppress CMAKE_OSX_SYSROOT if CFLAGS have -isysroot (#14324)
95f78e0 OS X: Search for SDK based on deployment target (#14324)