Fix installer deps

Fix installer deps

Just including .openconnect.exe.d from the main Makefile means it always
has to be up to date. Which means we always have to *build* openconnect,
even if it was just invoked for 'make clean'. That's bogus.

So shift the DLL dependency tracking out into its own Makefile, to be
invoked recursively. Much as I hate recursive make in the general case,
this is one of the few cases where it makes sense.

The automatic dependencies weren't working very well anyway, as it was
happy to 'discover' openconnect.exe and libopenconnect-5.dll in the MinGW
sysroot instead of using the locally built ones, *and* libtool messes up
the filenames so you can't just depend on .libs/openconnect.exe anyway.

Fix up $(DISTCLEANFILES) to be a bit more complete while we're at it.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>