Autoconf <-> Makefile overhaul

There's some terrifically b0rken behaviors we should fix (probably after next release) in our build process, in that several things happen in ./configure that definitely should be in make, and perhaps some case of vice versa (although I'm not sure I can think of anything).

Here's how it is supposed to be in my firm opinion:

• ./configure should set things up for the system you build on. That is, check arch, CPU type, available libs, handle options like --with-openmpi or "systemwide" stuff and so on. Most is about the system and very little is about JtR Jumbo. It should definitely not handle anything about checking Jumbo plugins or dependencies (that's what a makefile is all about!).
• make should merely build any files missing or not up to date according to whatever spec came from last ./configure.

If we get this straight and you do a git pull, ideally you shouldn't ever have to re-config or make clean. Only make. That's even if you got a few new plugin formats with that git update.
However, if you just installed a GPU and OpenCL (and didn't have that before) or decide to try out the rexgen lib, you'd need a re-conf and full rebuild.

To do:

• The plugin handling done in ./configure should be in Makefile just like it was before we added autoconf to Jumbo. We got too excited and tried to move just about everything from Makefile to autoconf, and ended up with rather silly behavior ever since.
• The dependency handling (currently plugin_deps.pl) happens in ./configure while it should happen during make. Moreover, I believe that script should possibly be ditched and we should learn how to do it with make and/or gcc themselves (I can't recall the details but a deps: target is often involved).
• The creation of dynamic_big_crypt.c from dynamic_big_crypt_header.cin and dynamic_big_crypt_hash.cin happens in ./configure while it too should totally be in Makefile.
• Probably some more things, add here...

Start by comparing the current plugin handling and dependency flow between ./configure and Makefile, including plugin_deps.pl. Then trace how dynamic_big_crypt.c is created from dynamic_big_crypt_header.cin and dynamic_big_crypt_hash.cin. Done means system setup remains in ./configure while plugin checks, dependencies, and generated files are handled during make without unnecessary reconfiguration or cleaning.