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.

pattern.c

@@ -363,7 +363,7 @@ static int compute_rowdata(int *ret, unsigned char *start, int len, int step)
 #define STILL_UNKNOWN 3
 
 #ifdef STANDALONE_SOLVER
-bool verbose = false;
+static bool verbose = false;
 #endif
 
 static bool do_recurse(unsigned char *known, unsigned char *deduced,
@@ -724,7 +724,7 @@ static unsigned char *generate_soluble(random_state *rs, int w, int h)
 #endif
 
 #ifdef STANDALONE_PICTURE_GENERATOR
-unsigned char *picture;
+static unsigned char *picture;
 #endif
 
 static char *new_game_desc(const game_params *params, random_state *rs,