custom-load.el and MH-E compatibility macros

[Stephen J. Turnbull]

mike.kupfer at xemacs.org writes:
 > We'll get the right results if defcustom-mh et al are already defined,
 > but nothing if they're not defined.

 > I thought about moving the deffoo-mh definitions to their own file, and
 > modifying the build rules to load the macros before invoking
 > Custom-make-dependencies.  For example, if I define a new makefile macro
 > called CUSTOM_LOAD_PREMAKE

This seems the most sensible way to proceed, except that I would
prefer that the Makefile variable be named CUSTOM_LOAD_PRELOADS.