fluidsynth: Fix CMake config

Upstream is concatenating CMAKE_INSTALL_LIBDIR onto CMAKE_INSTALL_PREFIX to get
a directory to use for the install_name on Darwin. This is not the right way of
making CMAKE_INSTALL_LIBDIR absolute.

The config was first set up for breakage when the CMAKE_INSTALL_LIBDIR was made relative,
in order to work around the wrong concatenation. This caused the _IMPORT_PREFIX to be
computed by walking up the directory tree from the config file, instead of using
the absolute CMAKE_INSTALL_LIBDIR.

Then outputs were introduced to the derivation and the config was moved to a different
output. Now the prefix it walks up to is the wrong prefix, and the config is busted.

Fix by reverting the original workaround so we have an absolute CMAKE_INSTALL_LIBDIR,
and pulling a patch that fixes the bad concatenation.

pkgs/applications/audio/fluidsynth/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, lib, fetchFromGitHub, buildPackages, pkg-config, cmake
+{ stdenv, lib, fetchFromGitHub, fetchpatch, buildPackages, pkg-config, cmake
 , alsa-lib, glib, libjack2, libsndfile, libpulseaudio
 , AppKit, AudioUnit, CoreAudio, CoreMIDI, CoreServices
 }:
@@ -14,6 +14,16 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-BSJu3jB7b5G2ThXBUHUNnBGl55EXe3nIzdBdgfOWDSM=";
   };
 
+  patches = [
+    # Fixes bad CMAKE_INSTALL_PREFIX + CMAKE_INSTALL_LIBDIR concatenation for Darwin install name dir
+    # Remove when PR merged & in release
+    (fetchpatch {
+      name = "0001-Fix-incorrect-way-of-turning-CMAKE_INSTALL_LIBDIR-absolute.patch";
+      url = "https://github.com/FluidSynth/fluidsynth/pull/1261/commits/03cd38dd909fc24aa39553d869afbb4024416de8.patch";
+      hash = "sha256-nV+MbFttnbNBO4zWnPLpnnEuoiESkV9BGFlUS9tQQfk=";
+    })
+  ];
+
   outputs = [ "out" "dev" "man" ];
 
   nativeBuildInputs = [ buildPackages.stdenv.cc pkg-config cmake ];
@@ -24,8 +34,6 @@ stdenv.mkDerivation rec {
 
   cmakeFlags = [
     "-Denable-framework=off"
-    # set CMAKE_INSTALL_NAME_DIR to correct value on darwin
-    "-DCMAKE_INSTALL_LIBDIR=lib"
   ];
 
   meta = with lib; {