C99 reserves the mem* namespace for future expansion. Some Rockbox
targets had issues with memswap() conflicting with another definition,
so fix that.

This adds an extra parameter to move_cursor() that's an optional pointer
to a bool indicating whether the cursor is visible.  This allows for
centralising the common idiom of having the keyboard cursor become
visible when a cursor key is pressed.  Consistently with the vast
majority of existing puzzles, the cursor moves even if it was invisible
before, and becomes visible even if it can't move.

The function now also returns one of the special constants that can be
returned by interpret_move(), so that the caller can correctly return
MOVE_UI_UPDATE or MOVE_NO_EFFECT without needing to carefully check for
changes itself.

Callers are updated only to the extent that they all pass NULL as the
new argument.  Most of them could now be substantially simplified.

I'm going to want to reuse it for sqrt(5) as well as sqrt(3) soon.

These allow for distinguishing the case where a puzzle doesn't have a
use for a key from the case where it just happens to have no effect in
the current state of the puzzle.  These were both represented by NULL,
but that now represents the case where a puzzle hasn't been updated to
make the distinction yet.

All the other constants named UI_* are special key names that can be
passed to midend_process_key(), but UI_UPDATE is a special return value
from the back-end interpret_move() function instead.  This renaming
makes the distinction clear and provides a naming convention for future
special return values from interpret_move().

Just noticed that if prefs_dir() returns NULL, we'll already have
passed it to this function before the calling functions get round to
checking. I think it's less wordy all round to make this helper
function propagate NULL than to mess about with lots of extra if
statements in between all the calls.

Finally, some user-visible behaviour changes as a payoff for all that
preparation work! In this commit, the GTK puzzles get a 'Preferences'
option in the menu, which presents a dialog box to configure the
preference settings.

On closing that dialog box, the puzzle preferences are enacted
immediately, and also saved to a configuration file where the next run
of the same puzzle will reload them.

The default file location is ~/.config/sgt-puzzles/<puzzlename>.conf,
although you can override the .config dir via $XDG_CONFIG_HOME or
override the Puzzles-specific subdir with $SGT_PUZZLES_DIR.

This is the first commit that actually exposes all the new preferences
work to the user, and therefore, I've also added documentation of all
the current preference options.

Environment variables that set specific settings of particular
puzzles, such as SLANT_SWAP_BUTTONS, LIGHTUP_LIT_BLOBS and
LOOPY_AUTOFOLLOW, now all affect the game behaviour via fields in
game_ui instead of being looked up by getenv in the individual
functions that need to know them.

The purpose of this refactoring is to put those config fields in a
place where other more user-friendly configuration systems will also
be able to access them, once I introduce one. However, for the moment,
there's no functional change: I haven't _yet_ changed how the user
sets those options. They're still set by environment variables alone.
All I'm changing here is where the settings are stored inside the
code, and exactly when they're read out of the environment to put into
the game_ui.

Specifically, the getenvs now happen during new_ui(). Or rather, in
all the puzzles I've changed here, they happen in a subroutine
legacy_prefs_override() called from within new_ui(), after it's set up
the default values for the settings, and then gives the environment a
chance to override them. Or rather, legacy_prefs_override() only
actually calls getenv the first time, and after that, it's cached the
answers it got.

In order to make the override functions less wordy, I've altered the
prototype of getenv_bool so that it returns an int rather than a bool,
and takes its default return value in the same form. That way you can
set the default to something other than 0 or 1, and find out whether a
value was present at all.

This commit only touches environment configuration specific to an
individual puzzle. The midend also has some standard environment-based
config options that apply to all puzzles, such as colour scheme and
default presets and preset-menu extension. I haven't handled those
yet.

This fixes a build failure introduced by commit 2e48ce132e011e8
yesterday.

When I saw that commit I expected the most likely problem would be in
the NestedVM build, which is currently the thing with the most most
out-of-date C implementation. And indeed the NestedVM toolchain
doesn't have <tgmath.h> - but much more surprisingly, our _Windows_
builds failed too, with a compile error inside <tgmath.h> itself!

I haven't looked closely into the problem yet. Our Windows builds are
done with clang, which comes with its own <tgmath.h> superseding the
standard Windows one. So you'd _hope_ that clang could make sense of
its own header! But perhaps the problem is that this is an unusual
compile mode and hasn't been tested.

My fix is to simply add a cmake check for <tgmath.h> - which doesn't
just check the file's existence, it actually tries compiling a file
that #includes it, so it will detect 'file exists but is mysteriously
broken' just as easily as 'not there at all'. So this makes the builds
start working again, precisely on Ben's theory of opportunistically
using <tgmath.h> where possible and falling back to <math.h>
otherwise.

It looks ugly, though! I'm half tempted to make a new header file
whose job is to include a standard set of system headers, just so that
that nasty #ifdef doesn't have to sit at the top of almost all the
source files. But for the moment this at least gets the build working
again.

C89 provided only double-precision mathematical functions (sin() etc),
and so despite using single-precision elsewhere, those are what Puzzles
has traditionally used.  C99 introduced single-precision equivalents
(sinf() etc), and I hope it's been long enough that we can safely use
them.  Maybe they'll even be faster.

Rather than directly use the single-precision functions, though, we use
the magic macros from <tgmath.h> that automatically choose the precision
of mathematical functions based on their arguments.  This has the
advantage that we only need to change which header we include, and thus
that we can switch back again if some platform has trouble with the new
header.