Add reference to emscripten libc issue

Author: hoodmane

test/test_projects/c.py

@@ -51,6 +51,7 @@
 
 libraries = []
 # Emscripten fails if you pass -lc...
+# See: https://github.com/emscripten-core/emscripten/issues/16680
 if sys.platform.startswith('linux') and "emscripten" not in os.environ.get("_PYTHON_HOST_PLATFORM", ""):
     libraries.extend(['m', 'c'])