Permit building GUI helper tools.

These look like puzzles, in that they link against a frontend and
provide the usual 'struct game', but they don't count as a puzzle for
purposes of shipping, or even having to have descriptions and icons.

There's one of these buried in the code already under an ifdef, which
I'll re-enable in the next commit.

cmake/platforms/emscripten.cmake

@@ -35,11 +35,15 @@ set(CMAKE_C_LINK_FLAGS "\
 -s EXTRA_EXPORTED_RUNTIME_METHODS='[cwrap,callMain]'")
 
 set(build_cli_programs FALSE)
+set(build_gui_programs FALSE)
 
 function(get_platform_puzzle_extra_source_files OUTVAR NAME)
   set(${OUTVAR} PARENT_SCOPE)
 endfunction()
 
+function(set_platform_gui_target_properties TARGET)
+endfunction()
+
 function(set_platform_puzzle_target_properties NAME TARGET)
   em_link_pre_js(${TARGET} ${CMAKE_SOURCE_DIR}/emccpre.js)
   em_link_js_library(${TARGET} ${CMAKE_SOURCE_DIR}/emcclib.js)