[2.7] Backport macOS universal build and installer fixes from 3.6. (GH-6469)

These include:

- bpo-32726: Provide an additional, more modern macOS installer variant that
  supports macOS 10.9+ systems in 64-bit mode only. Upgrade the supplied
  third-party libraries to OpenSSL 1.0.2n and SQLite 3.22.0. The 10.9+
  installer now supplies its own private copy of Tcl/Tk 8.6.8.

- bpo-24414: Default macOS deployment target is now set by ``configure`` to
  the build system's OS version (as is done by Python 3), not ``10.4``;
  override with, for example, ``./configure MACOSX_DEPLOYMENT_TARGET=10.4``.

- bpo-19019: All 2.7 macOS installer variants now supply their own version
  of ``OpenSSL 1.0.2``; the Apple-supplied SSL libraries and root
  certificates are not longer used.  The ``Installer Certificate`` command
  in ``/Applications/Python 2.7`` may be used to download and install a
  default set of root certificates from the third-party ``certifi`` package.

- bpo-11485: python.org macOS Pythons no longer supply a default SDK value
  (e.g. ``-isysroot /``) or specific compiler version default (e.g.
  ``gcc-4.2``) when building extension modules.  Use ``CC``, ``SDKROOT``,
  and ``DEVELOPER_DIR`` environment variables to override compilers or to
  use an SDK.  See Apple's ``xcrun`` man page for more info.

- prepare for pending Apple removal of 32-bit support in future macOS release

1 files changed, 9 insertions, 1 deletions

diff --git a/Mac/BuildScript/README.txt b/Mac/BuildScript/README.txt
index 6f6b20a013..32e7edc8f4 100644
--- a/Mac/BuildScript/README.txt
+++ b/Mac/BuildScript/README.txt
@@ -1,7 +1,15 @@
 Building a Python Mac OS X distribution
 =======================================
 
-The ``build-install.py`` script creates Python distributions, including
+WARNING
+-------
+
+The instructions in this README are incomplete and not up-to-date.
+In particular, they do not explain how to create a modern flat installer
+package from the now obsolete bundle-format installer package produced
+by ``build-installer.py``.
+
+The ``build-installer.py`` script creates Python distributions, including
 certain third-party libraries as necessary.  It builds a complete 
 framework-based Python out-of-tree, installs it in a funny place with 
 $DESTROOT, massages that installation to remove .pyc files and such, creates