synfig: pull upstream patch for autoconf-2.72 support

Without the change build fails against `autoconf-2.72` as:

    ./configure: line 22560: syntax error near unexpected token `;;'
    ./configure: line 22560: `             ;;'

pkgs/applications/graphics/synfigstudio/default.nix

@@ -1,6 +1,7 @@
 { lib
 , stdenv
 , fetchFromGitHub
+, fetchpatch
 , pkg-config
 , autoreconfHook
 , wrapGAppsHook
@@ -54,6 +55,17 @@ let
     pname = "synfig";
     inherit version src;
 
+    patches = [
+      # Pull upstream fix for autoconf-2.72 support:
+      #   https://github.com/synfig/synfig/pull/2930
+      (fetchpatch {
+        name = "autoconf-2.72.patch";
+        url = "https://github.com/synfig/synfig/commit/80a3386c701049f597cf3642bb924d2ff832ae05.patch";
+        stripLen = 1;
+        hash = "sha256-7gX8tJCR81gw8ZDyNYa8UaeZFNOx4o1Lnq0cAcaKb2I=";
+      })
+    ];
+
     sourceRoot = "${src.name}/synfig-core";
 
     configureFlags = [