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.

tree234.c

@@ -1501,12 +1501,12 @@ static void error(const char *fmt, ...) {
 }
 
 /* The array representation of the data. */
-void **array;
-int arraylen, arraysize;
-cmpfn234 cmp;
+static void **array;
+static int arraylen, arraysize;
+static cmpfn234 cmp;
 
 /* The tree representation of the same data. */
-tree234 *tree;
+static tree234 *tree;
 
 /*
  * Routines to provide a diagnostic printout of a tree. Currently