r14554: Write out header dependencies. This means all C files affected will be

r14554: Write out header dependencies. This means all C files affected will be
rebuilt when a header file is changed. It also means parallel builds work now.

It will take a minute or so to generate all the dependency information,
but there should be no need to rebuild that information later on, unless
a file changes.

This behaviour is only enabled when building in developer mode
(--enable-developer) and requires a GNU make (or compatible). In all other
cases, the file 'static_deps.mk' is included, which contains some basic
hardcoded dependency information.
(This used to be commit eb435386f015ce1d89eb6f7e7837622ebd9e1951)