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/library_dylink.js

@@ -3,6 +3,8 @@
 //
 // ==========================================================================
 
+var dlopenMissingError = "To use dlopen, you need to use Emscripten's linking support, see https://github.com/emscripten-core/emscripten/wiki/Linking"
+
 var LibraryDylink = {
 #if RELOCATABLE
   $resolveGlobalSymbol__deps: ['$asmjsMangle'],
@@ -182,14 +184,19 @@ 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); }],
+#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)