openalSolf: make it search its own $out for data files

Without those data files HRTF will silently fail to initialize.

It searches /usr and /usr/local by default but we don't have those paths.
It also searches XDG_DATA_DIRS but using that requires configuration by the
user. This patch makes makes it just work.

How to play with it:

- Build `mpv` with `openalSoft` support.

- cat << EOF > ~/.alsoftrc
[general]
hrtf = true
EOF

- Wear stereo headphones.

- Play a file with 6 or more channels with `mpv -ao openal $file`, e.g.
  https://archive.org/download/5.1SurroundSoundTestFilesVariousFormatsAACAC3MP4DTSWAV/5.1%20Surround%20Sound%20AAC%20Test.mp4

- Try `hrtf = false` to hear the difference.

pkgs/development/libraries/openal-soft/default.nix

@@ -20,6 +20,14 @@ stdenv.mkDerivation rec {
     sha256 = "0b0g0q1c36nfb289xcaaj3cmyfpiswvvgky3qyalsf9n4dj7vnzi";
   };
 
+  # this will make it find its own data files (e.g. HRTF profiles)
+  # without any other configuration
+  patches = [ ./search-out.patch ];
+  postPatch = ''
+    substituteInPlace Alc/helpers.c \
+      --replace "@OUT@" $out
+  '';
+
   nativeBuildInputs = [ cmake ];
 
   buildInputs = []

pkgs/development/libraries/openal-soft/search-out.patch

@@ -0,0 +1,12 @@
+diff -Nuar a/Alc/helpers.c b/Alc/helpers.c
+--- a/Alc/helpers.c	1970-01-01 00:00:01.000000000 +0000
++++ b/Alc/helpers.c	1970-01-01 00:00:02.000000000 +0000
+@@ -951,6 +951,8 @@
+             }
+         }
+ 
++        DirectorySearch("@OUT@/share", ext, &results);
++
+         alstr_reset(&path);
+     }
+