Dependency on lisp.h

[Eli Zaretskii]

I'd like to remind people who make _real_ changes in lisp.h (such as
struct member addition/removal, or function prototype declaration
changes due to addition/removal of arguments) to please touch config.h
so that everything is recompiled.

This is because lisp.h is not listed in dependencies (there are
reasons for this), so a simple "make" might not compile some of the
sources, and you won't see some of the incompatibilities introduced by
your change.  This could cause subtle bugs that take hours to debug.