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.

penrose.c

@@ -509,8 +509,8 @@ void penrose_calculate_size(int which, int tilesize, int w, int h,
 #include <stdio.h>
 #include <string.h>
 
-int show_recursion = 0;
-int ntiles, nfinal;
+static int show_recursion = 0;
+static int ntiles, nfinal;
 
 static int test_cb(penrose_state *state, vector *vs, int n, int depth)
 {