Fix missing statics and #includes on variables.

After Ben fixed all the unwanted global functions by using gcc's
-Wmissing-declarations to spot any that were not predeclared, I
remembered that clang has -Wmissing-variable-declarations, which does
the same job for global objects. Enabled it in -DSTRICT=ON, and made
the code clean under it.

Mostly this was just a matter of sticking 'static' on the front of
things. One variable was outright removed ('verbose' in signpost.c)
because after I made it static clang was then able to spot that it was
also unused.

The more interesting cases were the ones where declarations had to be
_added_ to header files. In particular, in COMBINED builds, puzzles.h
now arranges to have predeclared each 'game' structure defined by a
puzzle backend. Also there's a new tiny header file gtk.h, containing
the declarations of xpm_icons and n_xpm_icons which are exported by
each puzzle's autogenerated icon source file and by no-icon.c. Happily
even the real XPM icon files were generated by our own Perl script
rather than being raw xpm output from ImageMagick, so there was no
difficulty adding the corresponding #include in there.

unequal.c

@@ -1075,7 +1075,7 @@ static int gg_best_clue(game_state *state, int *scratch, digit *latin)
 }
 
 #ifdef STANDALONE_SOLVER
-int maxtries;
+static int maxtries;
 #define MAXTRIES maxtries
 #else
 #define MAXTRIES 50
@@ -2191,7 +2191,7 @@ const struct game thegame = {
 #include <time.h>
 #include <stdarg.h>
 
-const char *quis = NULL;
+static const char *quis = NULL;
 
 #if 0 /* currently unused */