Error at link time if dlfcn functions are used without MAIN_MODULE

Only if ALLOW_UNIMPLEMENTED_SYSCALLS (which is also set by STRICT)
is used.

This was previously only ever a runtime error.

Should help with #15276 (comment)

Author: sbc100

src/jsifier.js

@@ -322,7 +322,9 @@ function ${name}(${args}) {
         //   foo: '=[value]'
         //  emits
         //   'var foo = [value];'
-        if (typeof snippet === 'string' && snippet[0] == '=') snippet = snippet.substr(1);
+        if (typeof snippet === 'string' && snippet[0] == '=') {
+          snippet = snippet.substr(1);
+        }
         contentText = `var ${finalName} = ${snippet};`;
       }
       var sig = LibraryManager.library[ident + '__sig'];

src/library_dylink.js

@@ -3,6 +3,8 @@
 //
 // ==========================================================================
 
+var dlopenMissingError = "'To use dlopen, you need enable dynamic linking, see https://github.com/emscripten-core/emscripten/wiki/Linking'"
+
 var LibraryDylink = {
 #if RELOCATABLE
   $resolveGlobalSymbol__internal: true,
@@ -188,14 +190,24 @@ var LibraryDylink = {
 #endif
 
 #if MAIN_MODULE == 0
+#if !ALLOW_UNIMPLEMENTED_SYSCALLS
+  _dlopen_js__deps: [function() { error(dlopenMissingError); }],
+  _emscripten_dlopen_js__deps: [function() { error(dlopenMissingError); }],
+  _dlsym_js__deps: [function() { error(dlopenMissingError); }],
+#else
+  $dlopenMissingError: `= ${dlopenMissingError}`,
+  _dlopen_js__deps: ['$dlopenMissingError'],
+  _emscripten_dlopen_js__deps: ['$dlopenMissingError'],
+  _dlsym_js__deps: ['$dlopenMissingError'],
+#endif
   _dlopen_js: function(filename, flag) {
-    abort("To use dlopen, you need to use Emscripten's linking support, see https://github.com/emscripten-core/emscripten/wiki/Linking");
+    abort(dlopenMissingError);
   },
   _emscripten_dlopen_js: function(filename, flags, user_data, onsuccess, onerror) {
-    abort("To use dlopen, you need to use Emscripten's linking support, see https://github.com/emscripten-core/emscripten/wiki/Linking");
+    abort(dlopenMissingError);
   },
   _dlsym_js: function(handle, symbol) {
-    abort("To use dlopen, you need to use Emscripten's linking support, see https://github.com/emscripten-core/emscripten/wiki/Linking");
+    abort(dlopenMissingError);
   },
 #else // MAIN_MODULE != 0
   // dynamic linker/loader (a-la ld.so on ELF systems)