Use warn for printing #warning messages in JS libraries

Previously we were only writing the message to stderr, and not recording
it as an actual warning.  After this change, such messages are now
reported as warnings and controllable using `-Werror`, etc.

Author: sbc100

src/parseTools.mjs

@@ -180,7 +180,7 @@ export function preprocess(filename) {
           showStack.pop();
         } else if (first === '#warning') {
           if (showCurrentLine()) {
-            printErr(
+            warn(
               `${filename}:${i + 1}: #warning ${trimmed.substring(trimmed.indexOf(' ')).trim()}`,
             );
           }

test/test_other.py

@@ -4919,6 +4919,13 @@ def test_jslib_warnings(self):
     self.assertNotContained('This warning should not be present!', proc.stderr)
     self.assertContained('warning_in_js_libraries.js:5: #warning This is a warning string!', proc.stderr)
     self.assertContained('warning_in_js_libraries.js:7: #warning This is a second warning string!', proc.stderr)
+    self.assertContained('emcc: warning: warnings in JS library compilation [-Wjs-compiler]', proc.stderr)
+
+    err = self.expect_fail([EMCC, test_file('hello_world.c'), '--js-library', test_file('warning_in_js_libraries.js'), '-Werror'])
+    self.assertNotContained('This warning should not be present!', err)
+    self.assertContained('warning_in_js_libraries.js:5: #warning This is a warning string!', err)
+    self.assertContained('warning_in_js_libraries.js:7: #warning This is a second warning string!', err)
+    self.assertContained('emcc: error: warnings in JS library compilation [-Wjs-compiler] [-Werror]', err)
 
   # Tests using the #error directive in JS library files
   def test_jslib_errors(self):