Update docs for internal EMSCRIPTEN_VERSION setting. NFC (#25664)

I proposed removing this internal setting in #25661 but it turns out it
was useful for externally managed JS library files.

However, I don't think it should be tested as part of
`emscripten_get_compiler_setting.c` since C/C++ can already get the
compiler version using `VERSION` macros. . There should be no need for
the runtime checks.

Indeed I hope we can stop exposing any internal settings to
emscripten_get_compiler_setting: #25663

Also, add test for @juj's specific use case for `EMSCRIPTEN_VERSION` in
a JS library file.

Author: sbc100

src/settings_internal.js

@@ -107,7 +107,8 @@ var WASI_MODULE_NAME = "wasi_snapshot_preview1";
 var JS_LIBRARIES = [];
 
 // This will contain the emscripten version. This can be useful in combination
-// with RETAIN_COMPILER_SETTINGS
+// with external JS library files that need to check the version of emscripten
+// they are being used with.
 var EMSCRIPTEN_VERSION = '';
 
 // Will be set to 0 if -fno-rtti is used on the command line.

test/core/emscripten_get_compiler_setting.c

@@ -13,6 +13,6 @@ int main() {
   printf("INVOKE_RUN: %ld\n", emscripten_get_compiler_setting("INVOKE_RUN"));
   assert((unsigned)emscripten_get_compiler_setting("OPT_LEVEL") <= 3);
   assert((unsigned)emscripten_get_compiler_setting("DEBUG_LEVEL") <= 4);
-  printf("EMSCRIPTEN_VERSION: %s\n", (char*)emscripten_get_compiler_setting("EMSCRIPTEN_VERSION"));
+  printf("CLOSURE_WARNINGS: %s\n", (char*)emscripten_get_compiler_setting("CLOSURE_WARNINGS"));
 }
 

test/core/emscripten_get_compiler_setting.out

@@ -1,2 +1,2 @@
 INVOKE_RUN: 1
-EMSCRIPTEN_VERSION: waka
+CLOSURE_WARNINGS: quiet

test/test_core.py

@@ -1920,9 +1920,7 @@ def test_emscripten_get_now(self):
   def test_emscripten_get_compiler_setting(self):
     if not self.is_optimizing() and ('-flto' in self.cflags or '-flto=thin' in self.cflags):
       self.skipTest('https://github.com/emscripten-core/emscripten/issues/25015')
-    expected = read_file(test_file('core/emscripten_get_compiler_setting.out'))
-    expected = expected.replace('waka', utils.EMSCRIPTEN_VERSION)
-    self.do_runf('core/emscripten_get_compiler_setting.c', expected, cflags=['-sRETAIN_COMPILER_SETTINGS'])
+    self.do_core_test('emscripten_get_compiler_setting.c', cflags=['-sRETAIN_COMPILER_SETTINGS'])
 
   def test_emscripten_get_compiler_setting_error(self):
     # with assertions, a runtime error is shown if you try to use the API without RETAIN_COMPILER_SETTINGS

test/test_jslib.py

@@ -671,3 +671,11 @@ def test_jslib_override_system_symbol(self):
     # When WebGL is explicitly linked to in strict mode, the linking order on command line should enable overriding.
     self.cflags += ['-sAUTO_JS_LIBRARIES=0', '-sMAX_WEBGL_VERSION=2', '-lwebgl.js', '--js-library', test_file('test_jslib_override_system_symbol.js')]
     self.do_run_in_out_file_test('test_jslib_override_system_symbol.c')
+
+  def test_jslib_version_check(self):
+    create_file('libfoo.js', '''
+      #if parseInt(EMSCRIPTEN_VERSION.split('.')[0]) > 3
+      #error "library does not support emscripten > 3.0.0"
+      #endif
+    ''')
+    self.assert_fail([EMCC, '--js-library=libfoo.js'], 'error: libfoo.js:3: #error "library does not support emscripten > 3.0.0"')