build: add some warning cflags

These were in the old configure script too.

Two flags are explicitly tested, because I have no idea how widespread
support for them is, and testing them is just easier than trying to look
them up in various gcc/clang manuals. There are people using gcc 4.2
out there, so some caution is warranted.
old-build: drop support for anything but Linux, simplify

A lot of effort was spent on making the waf based build-system work
properly on all supported platforms, while the old configure script was
neglected. It seems that nobody maintains the non-Linux parts of the
configure script anymore, and all improvements go into the waf scripts.

Thus it makes no sense anymore to maintain the non-Linux parts. They're
just dead weight. Remove them completely.

Also apply some additional simplifications. For example, listing
enabled/disabled VO modules seems like a waste of effort.
old-build: accidental rewrite

This started as a bunch of smaller changes to make the old configure
script maintainable with minimum effort. It ended up as complete
rewrite, because at once point I started to like shell programming (I
hope this sickness is curable), and I wanted to see how small I can
make the configure script. The typical configure test is now 1 or 2
lines big, located in 1 or 2 places, instead of >15 lines and being
spread over 5 or 6 places.

The main "trick" is factoring the tests into a few generic, commonly
needed tests, instead of writing everything manually.
manpage: updates changes.rst

The situation has changed a bit since the days of mplayer2, so we can
use more/less diplomatic wording. Merge the two sections listing
changes from MPlayer and mplayer2. Mention the client API and Lua
scripting as alternatives to slave mode.

I'm calling MPlayer code "horrible". This is not meant as an offense,
but after turning around almost every line of MPlayer code, I believe
I have a right to say this. Sorry. I would say that MPlayer has a
surprisingly sane and simple architecture (for what it is), but much
of it drowned under a load of evil hacks or not-cleaned-up-yet code.
vf_vapoursynth: allow parallel processing

VapourSynth won't just filter multiple frames at once on its own. You
have to request multiple frames at once manually. This is what this
commit introduces: a sub-option controls how many frames will be
requested at once. This also changes the semantics of the maxbuffer sub-
option, now renamed to buffered-frames.