Fix bug after PR #4665 that rebuilding SDL2 with -s USE_SDL2=1 after emcc --clear-ports would fail.

Author: juj

src/modules.js

@@ -147,7 +147,7 @@ var LibraryManager = {
 
     // If there are any explicitly specified system JS libraries to link to, add those to link.
     if (SYSTEM_JS_LIBRARIES) {
-      libraries = libraries.concat(SYSTEM_JS_LIBRARIES);
+      libraries = libraries.concat(SYSTEM_JS_LIBRARIES.split(','));
     }
 
     libraries = libraries.concat(additionalLibraries);

tools/shared.py

@@ -2028,7 +2028,7 @@ def path_to_system_js_libraries_for_settings(link_settings):
     if 'LZ4=1' in link_settings: system_js_libraries += ['library_lz4.js']
     if 'USE_SDL=1' in link_settings: system_js_libraries += ['library_sdl.js']
     if 'USE_SDL=2' in link_settings: system_js_libraries += ['library_egl.js', 'library_glut.js', 'library_gl.js']
-    return system_js_libraries
+    return map(lambda x: path_from_root('src', x), system_js_libraries)
 
 # compatibility with existing emcc, etc. scripts
 Cache = cache.Cache(debug=DEBUG_CACHE)