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.

cmake/platforms/unix.cmake

@@ -63,7 +63,7 @@ if(STRICT AND (CMAKE_C_COMPILER_ID MATCHES "GNU"))
 endif()
 
 if(STRICT AND (CMAKE_C_COMPILER_ID MATCHES "Clang"))
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wmissing-prototypes")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wmissing-prototypes -Wmissing-variable-declarations")
 endif()
 
 add_compile_definitions(HELP_DIR="${CMAKE_INSTALL_PREFIX}/share/sgt-puzzles/help")