Issue #13950: Improve support for OS X Xcode 4:

Issue #13950: Improve support for OS X Xcode 4:
Fix ./configure to provide a more sensible default for
MACOSX_DEPLOYMENT_TARGET. Currently the default is the minimum OS X level
that supports the universal arch option, typically 10.4, even when no
universalsdk is selected. This causes various desirable features that
depend on later OS X versions, like libedit readline support, to be
omitted from the build. A more complete solution would take into account
the SDK that is being used and better tailor the universal arch options.
For now, change the existing tests to only apply to build systems of 10.5
and earlier; for 10.6 and later, use the build system version as the
default deployment target if the MACOSX_DEPLOYMENT_TARGET environment
variable is not provided.