Avoid compiling stb_image at link time (#13658)

Instead make it into a system library like libfetch or libembind.
This was the final example of a place where emcc was inject extra
source files into the build at link time.

emcc.py

@@ -1524,12 +1524,11 @@ There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR P
       shared.Settings.WORKAROUND_IOS_9_RIGHT_SHIFT_BUG = 0
       shared.Settings.WORKAROUND_OLD_WEBGL_UNIFORM_UPLOAD_IGNORED_OFFSET_BUG = 0
 
+    forced_stdlibs = []
+
     if shared.Settings.STB_IMAGE and final_suffix in EXECUTABLE_ENDINGS:
-      input_files.append((len(newargs), shared.path_from_root('third_party', 'stb_image.c')))
+      forced_stdlibs.append('libstb_image')
       shared.Settings.EXPORTED_FUNCTIONS += ['_stbi_load', '_stbi_load_from_memory', '_stbi_image_free']
-      # stb_image 2.x need to have STB_IMAGE_IMPLEMENTATION defined to include the implementation
-      # when compiling
-      newargs.append('-DSTB_IMAGE_IMPLEMENTATION')
 
     if shared.Settings.USE_WEBGL2:
       shared.Settings.MAX_WEBGL_VERSION = 2
@@ -1537,8 +1536,6 @@ There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR P
     if not shared.Settings.GL_SUPPORT_SIMPLE_ENABLE_EXTENSIONS and shared.Settings.GL_SUPPORT_AUTOMATIC_ENABLE_EXTENSIONS:
       exit_with_error('-s GL_SUPPORT_SIMPLE_ENABLE_EXTENSIONS=0 only makes sense with -s GL_SUPPORT_AUTOMATIC_ENABLE_EXTENSIONS=0!')
 
-    forced_stdlibs = []
-
     if shared.Settings.ASMFS and final_suffix in EXECUTABLE_ENDINGS:
       forced_stdlibs.append('libasmfs')
       shared.Settings.FILESYSTEM = 0

system/lib/stb_image.c

@@ -0,0 +1,9 @@
+// Copyright 2021 The Emscripten Authors.  All rights reserved.
+// Emscripten is available under two separate licenses, the MIT license and the
+// University of Illinois/NCSA Open Source License.  Both these licenses can be
+// found in the LICENSE file.
+
+// stb_image 2.x need to have STB_IMAGE_IMPLEMENTATION defined to include
+// the implementation when compiling
+#define STB_IMAGE_IMPLEMENTATION
+#include <stb_image.h>

tools/system_libs.py

@@ -1159,6 +1159,15 @@ class libfetch(MTLibrary):
     return [shared.path_from_root('system', 'lib', 'fetch', 'emscripten_fetch.cpp')]
 
 
+class libstb_image(Library):
+  name = 'libstb_image'
+  never_force = True
+  includes = [['third_party']]
+
+  def get_files(self):
+    return [shared.path_from_root('system', 'lib', 'stb_image.c')]
+
+
 class libasmfs(MTLibrary):
   name = 'libasmfs'
   never_force = True