gst_all_1.gst-plugins-bad: make openh264 optional

This fixes building gst-plugins-bad for RISC-V, where openh264 is
unavailable.

pkgs/development/libraries/gstreamer/bad/default.nix

···
 , neon
 , openal
 , openexr_3
+, openh264Support ? lib.meta.availableOn stdenv.hostPlatform openh264
 , openh264
 , libopenmpt
 , pango
···
     neon
     openal
     openexr_3
-    openh264
     rtmpdump
     pango
     soundtouch
···
     bluez
   ] ++ lib.optionals microdnsSupport [
     libmicrodns
+  ] ++ lib.optionals openh264Support [
+    openh264
   ] ++ lib.optionals (gst-plugins-base.waylandEnabled && stdenv.isLinux) [
     libva # vaapi requires libva -> libdrm -> libpciaccess, which is Linux-only in nixpkgs
     wayland
···
     "-Daja=disabled" # should pass libajantv2 via aja-sdk-dir instead
     "-Dmicrodns=${if microdnsSupport then "enabled" else "disabled"}"
     "-Dbluez=${if bluezSupport then "enabled" else "disabled"}"
+    (lib.mesonEnable "openh264" openh264Support)
     (lib.mesonEnable "doc" enableDocumentation)
   ]
   ++ lib.optionals (!stdenv.isLinux) [