Support Core Text on earlier versions of Mac OS X

Support Core Text on earlier versions of Mac OS X

Loosely based on behdad/harfbuzz@b96af03c20e46105982b3608b608614403540661.

Prefer to link against ApplicationServices to maximize the
portability of binaries built on newer versions of macOS.

The symbol kCTFontURLAttribute, which is checked in this commit, was
introduced in Mac OS X 10.6, the latest of any Core Text symbols that
we use. It is essential to our Core Text font provider, so this is the
earliest version of Mac OS X where we can support this font provider.

The TARGET_OS_IPHONE conditional that this commit adds is necessary to
continue supporting iOS in addition to supporting old Mac OS X. On iOS,
CoreText.h *must* be included to use Core Text, whereas on old Mac OS X,
CoreText.h is not directly accessible and ApplicationServices.h must be
used. On modern macOS, either header works. This conditional is also
used in HarfBuzz.