Make configure script use pkg-config to detect the location of Tcl/Tk
headers and libraries, used to build tkinter.

On macOS, a Tcl/Tk configuration provided by pkg-config will be preferred
over Tcl/Tk frameworks installed in ``/{System/,}Library/Frameworks``.
If both exist and the latter is preferred, the appropriate
``--with-tcltk-*`` configuration options need to be explicitly set.