libsixel: make linking against gd a choosable option. (#372698)

pkgs/by-name/li/libsixel/package.nix

@@ -7,19 +7,33 @@
   gdk-pixbuf,
   gd,
   pkg-config,
+
+  # Enable linking against image loading libraries as part of the
+  # implementation of the sixel_helper_{load,write}_image_file() functions.
+  # These helper functions are not needed for the main functionality of the
+  # library to encode image buffers to sixels.
+  #
+  # libsixel already uses vendored stb image loading to provide basic
+  # implementations, but also allows for the "gd" library to be linked for
+  # a wider set of image formats.
+  # This pulls in a large amount of deps bloating the resulting library.
+  #
+  # Default off, but configurable in case you really need it.
+  withGd ? false,
 }:
-stdenv.mkDerivation rec {
+
+stdenv.mkDerivation (finalAttrs: {
   pname = "libsixel";
   version = "1.10.5";
 
   src = fetchFromGitHub {
     owner = "libsixel";
     repo = "libsixel";
-    rev = "v${version}";
+    rev = "v${finalAttrs.version}";
     hash = "sha256-obzBZAknN3N7+Bvtd0+JHuXcemVb7wRv+Pt4VjS6Bck=";
   };
 
-  buildInputs = [
+  buildInputs = lib.optionals withGd [
     gdk-pixbuf
     gd
   ];
@@ -34,8 +48,14 @@ stdenv.mkDerivation rec {
 
   mesonFlags = [
     "-Dtests=enabled"
-    # build system seems to be broken here, it still seems to handle jpeg
-    # through some other ways.
+    "-Dimg2sixel=enabled"
+    "-Dsixel2png=enabled"
+
+    (lib.mesonEnable "gd" withGd)
+
+    # build system seems to be broken here; error message indicates pkconfig
+    # issue.
+    # Not to worry: jpeg and png are handled by the built-in stb and/or gd lib.
     "-Djpeg=disabled"
     "-Dpng=disabled"
   ];
@@ -47,4 +67,4 @@ stdenv.mkDerivation rec {
     license = licenses.mit;
     platforms = platforms.unix;
   };
-}
+})