Use emscripten's "official" SDL2 port.

Instead of building ourselves.

pkgdef/emscripten.sh

@@ -1,5 +1,20 @@
 description "Emscripten build environment"
 
+SDL2_PACKAGE_CONFIG="
+prefix=/
+exec_prefix=/
+libdir=/
+includedir=/
+
+Name: sdl2
+Description: sdl2
+Version: 2.20.0
+Requires:
+Conflicts:
+Libs: -sUSE_SDL=2 -flto -sASYNCIFY -sENVIRONMENT=web
+Cflags: -sUSE_SDL=2
+"
+
 do_fetch() {
     check_tool emcc
     mkdir -p "$PACKAGE_BUILD_DIR"
@@ -17,18 +32,26 @@ make_wrapper_script() {
     path="$PACKAGE_INSTALL_DIR/bin/wasm32-unknown-emscripten-$toolname"
     mkdir -p "$PACKAGE_INSTALL_DIR/bin"
     (echo "#!/bin/bash"
-     echo ". $PACKAGE_BUILD_DIR/emsdk_env.sh >/dev/null"
-     echo "export EMCC_CFLAGS='-s ERROR_ON_MISSING_LIBRARIES=1'"
+     echo "export EM_CACHE=$PACKAGE_INSTALL_DIR/emscripten_cache"
+     echo "export EM_FROZEN_CACHE="
      echo "exec $emtoolname \"\$@\"") > "$path"
     chmod a+rx "$path"
 }
 
+do_install() {
+    # Emscripten has its own patched version of SDL2 that will get installed
+    # automatically if -sUSE_SDL=2 is provided on the command line. By
+    # installing a "fake" pkg-config file that just supplies this argument,
+    # we short-circuit the build for the normal SDL package.
+    mkdir -p "$PACKAGE_INSTALL_DIR/lib/pkgconfig"
+    echo "$SDL2_PACKAGE_CONFIG" > "$PACKAGE_INSTALL_DIR/lib/pkgconfig/sdl2.pc"
+    # TODO: Add pkg-config files for other libraries emscripten supports.
+
     # Emscripten is a weird beast, and wants us to use all its weird tools
     # (emconfigure, emmake, etc.). Instead of that, we define a "fake" system
     # named wasm32-unknown-emscripten and create wrapper scripts for all the
     # necessary compile tools. Then we can build like any other normal
-# cross-compile system.
-do_install() {
+    # cross-compile system -- mostly.
     make_wrapper_script ar     emar
     make_wrapper_script gcc    emcc
     make_wrapper_script g++    em++