guitarix: fix build with lv2 1.18, enable faust plugins

pkgs/applications/audio/guitarix/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, gettext, intltool, pkgconfig, python2
+{ stdenv, fetchurl, fetchpatch, faust, gettext, intltool, pkgconfig, python2
 , avahi, bluez, boost, eigen, fftw, glib, glib-networking
 , glibmm, gsettings-desktop-schemas, gtkmm2, libjack2
 , ladspaH, libav, libsndfile, lilv, lrdf, lv2, serd, sord, sratom
@@ -19,7 +19,15 @@ stdenv.mkDerivation rec {
     sha256 = "1nn80m1qagfhvv69za60f0w6ck87vmk77qmqarj7fbr8avwg63s9";
   };
 
-  nativeBuildInputs = [ gettext intltool wrapGAppsHook pkgconfig python2 wafHook ];
+  patches = [
+    (fetchpatch {
+      url = "https://git.archlinux.org/svntogit/community.git/plain/trunk/guitarix-0.39.0-fix_faust_and_lv2_plugins.patch?id=8579b4dfe85e04303ad2d9771ed699f04ea7b7cf";
+      stripLen = 1;
+      sha256 = "0pgkhi4v4vrzjnig0ggmz207q4x5iyk2n6rjj8s5lv15fia7qzp4";
+    })
+  ];
+
+  nativeBuildInputs = [ faust gettext intltool wrapGAppsHook pkgconfig python2 wafHook ];
 
   buildInputs = [
     avahi bluez boost eigen fftw glib glibmm glib-networking.out
@@ -28,11 +36,16 @@ stdenv.mkDerivation rec {
     zita-resampler curl
   ];
 
+  postPatch = ''
+    # Fix build with lv2 1.18: https://github.com/brummer10/guitarix/commit/c0334c72
+    find . -type f -exec fgrep -q LV2UI_Descriptor {} \; \
+      -exec sed -i {} -e 's/const struct _\?LV2UI_Descriptor/const LV2UI_Descriptor/' \;
+  '';
+
   wafConfigureFlags = [
     "--shared-lib"
     "--no-desktop-update"
     "--enable-nls"
-    "--no-faust" # todo: find out why --faust doesn't work
     "--install-roboto-font"
     "--includeresampler"
     "--convolver-ffmpeg"