js: Add a CMake variable to control whether Emscripten emits WASM

I've finally got bored of keeping (and occasionally losing) a patch
that turns it off unconditionally.

cmake/platforms/emscripten.cmake

@@ -3,6 +3,9 @@ set(platform_gui_libs)
 set(platform_libs)
 set(CMAKE_EXECUTABLE_SUFFIX ".js")
 
+set(WASM ON
+  CACHE BOOL "Compile to WebAssembly rather than plain JavaScript")
+
 set(emcc_export_list
   # Event handlers for mouse and keyboard input
   _mouseup
@@ -33,6 +36,11 @@ set(CMAKE_C_LINK_FLAGS "\
 -s ALLOW_MEMORY_GROWTH=1 \
 -s EXPORTED_FUNCTIONS='[${emcc_export_string}]' \
 -s EXTRA_EXPORTED_RUNTIME_METHODS='[cwrap,callMain]'")
+if(WASM)
+  set(CMAKE_C_LINK_FLAGS "${CMAKE_C_LINK_FLAGS} -s WASM=1")
+else()
+  set(CMAKE_C_LINK_FLAGS "${CMAKE_C_LINK_FLAGS} -s WASM=0")
+endif()
 
 set(build_cli_programs FALSE)
 set(build_gui_programs FALSE)