libwebsockets: improve cross-compilation support

Building works on aarch64-multiplatform:

- libwebsockets_3_1
- libwebsockets_3_2
- libwebsockets_4_0
- libwebsockets_4_1

pkgs/development/libraries/libwebsockets/default.nix

@@ -20,7 +20,7 @@ let
       "-DLWS_WITH_PLUGINS=ON"
       "-DLWS_WITH_IPV6=ON"
       "-DLWS_WITH_SOCKS5=ON"
-    ];
+    ] ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) "-DLWS_WITHOUT_TESTAPPS=ON";
 
     NIX_CFLAGS_COMPILE = lib.optionalString stdenv.cc.isGNU "-Wno-error=unused-but-set-variable";