Issue #1584: Provide options to override default search paths for Tcl and Tk
when building _tkinter.  configure has two new options; if used, both must
be specified:

  ./configure \
      --with-tcltk-includes="-I/opt/local/include" \
      --with-tcltk-libs="-L/opt/local/lib -ltcl8.5 -ltk8.5"

In addition, the options can be overridden with make:

   make \
       TCLTK_INCLUDES="-I/opt/local/include" \
       TCLTK_LIBS="-L/opt/local/lib -ltcl8.6 -ltk8.6"
Issue #15663: Tcl/Tk 8.5.15 is now included with the OS X 10.6+
64-bit/32-bit installer for 10.6+.  It is no longer necessary
to install a third-party version of Tcl/Tk 8.5 to work around the
problems in the Apple-supplied Tcl/Tk 8.5 shipped in OS X 10.6
and later releases.
Issue #19019: Change the OS X installer build script to use CFLAGS instead
of OPT for special build options.  By setting OPT, some compiler-specific
options like -fwrapv were overridden and thus not used, which could result
in broken interpreters when building with clang.
Sync 2.7.x, 3.3.x, and 3.4.x versions of OS X build-installer.py.