patch-ac (from mrg@NetBSD.org):

Look for libGL.so, not any particular libGL.so.<X>, on NetBSD.
(the comment inline explains why)

src/video/x11/SDL_x11gl.c

@@ -35,6 +35,9 @@
 #define DEFAULT_OPENGL	"libGL.so.3"
 #elif defined(__OpenBSD__)
 #define DEFAULT_OPENGL	"libGL.so.4.0"
+#elif defined(__NetBSD__)
+/* pkgsrc is .1, base system is .2 */
+#define DEFAULT_OPENGL	"libGL.so"
 #else
 #define DEFAULT_OPENGL	"libGL.so.1"
 #endif