Issue #12627: Implement PEP 394 for OS X framework builds.

Issue #12627: Implement PEP 394 for OS X framework builds.
OS X framework builds already created versioned symlinks for all
executables and scripts installed in the framework bin directory,
of the general form ${cmd} - ${cmd}2.7.  The changes here add a
hierarchy of ${cmd} -> ${cmd}2 -> ${cmd}2.7.  Per previous
practice, all of the links are created in the framework bin
directory for both the install and altinstall targets.  This is
consistent with the long-standing recommendation to manage multiple
framework versions by adding and ordering framework bin directories
on $PATH.  Also, per past practice, symlinks to all framework bin
entries are created in $prefix/bin (by default, /usr/local/bin)
for the install target and only versioned links are created for
altinstall, although the use of these links is not recommended
for framework builds and their installation is optional with
the standard OS X installers.