bpo-34956: Fix macOS _tkinter use of Tcl/Tk in /Library/Frameworks (GH-20171)

_tkinter now builds and links with non-system Tcl and Tk frameworks if they
are installed in /Library/Frameworks as had been the case on older releases
of macOS. If a macOS SDK is explicitly configured, by using ./configure
--enable-universalsdk= or -isysroot, only a Library/Frameworks directory in
the SDK itself is searched. The default behavior can still be overridden with
configure --with-tcltk-includes and --with-tcltk-libs.