Fix compiler warning on macOS

Compiling drivers/Posix/Fl_Posix_System_Driver.cxx...
drivers/Posix/Fl_Posix_System_Driver.cxx:176:14: warning: unused function 'quadruple_dlopen' [-Wunused-function]

src/drivers/Posix/Fl_Posix_System_Driver.cxx

@@ -172,7 +172,8 @@ int Fl_Posix_System_Driver::run_program(const char *program, char **argv, char *
 }
 
 
-#if HAVE_DLSYM && HAVE_DLFCN_H
+#if HAVE_DLSYM && HAVE_DLFCN_H && !defined (__APPLE_CC__)
+
 static void* quadruple_dlopen(const char *libname)
 {
   char filename2[FL_PATH_MAX];