Remove testing instruction (see bug #5283)

src/thread/pthread/SDL_systhread.c

@@ -41,8 +41,6 @@
 #include "../../core/linux/SDL_dbus.h"
 #endif /* __LINUX__ */
 
-#undef HAVE_DLOPEN
-
 #if (defined(__LINUX__) || defined(__MACOSX__) || defined(__IPHONEOS__)) && defined(HAVE_DLOPEN)
 #include <dlfcn.h>
 #ifndef RTLD_DEFAULT