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.

gtk.c

@@ -30,6 +30,7 @@
 #include <X11/Xatom.h>
 
 #include "puzzles.h"
+#include "gtk.h"
 
 #if GTK_CHECK_VERSION(2,0,0)
 # define USE_PANGO
@@ -2391,8 +2392,8 @@ static void menu_preset_event(GtkMenuItem *menuitem, gpointer data)
     midend_redraw(fe->me);
 }
 
-GdkAtom compound_text_atom, utf8_string_atom;
-bool paste_initialised = false;
+static GdkAtom compound_text_atom, utf8_string_atom;
+static bool paste_initialised = false;
 
 static void set_selection(frontend *fe, GdkAtom selection)
 {
@@ -3076,9 +3077,6 @@ static void menu_about_event(GtkMenuItem *menuitem, gpointer data)
     "version", ver,                                                 \
     "comments", "Part of Simon Tatham's Portable Puzzle Collection"
 
-    extern char *const *const xpm_icons[];
-    extern const int n_xpm_icons;
-
     if (n_xpm_icons) {
         GdkPixbuf *icon = gdk_pixbuf_new_from_xpm_data
             ((const gchar **)xpm_icons[0]);
@@ -3193,8 +3191,6 @@ static frontend *new_window(
     GList *iconlist;
     int x, y, n;
     char errbuf[1024];
-    extern char *const *const xpm_icons[];
-    extern const int n_xpm_icons;
     struct preset_menu *preset_menu;
 
     fe = snew(frontend);