csound: fix cross with gettext hack

We need to ignore gettext in CMAKE_PREFIX_PATH on cross to prevent
`find_program` from picking up the wrong gettext. See the `webkitgtk`
and `fish` derivations for similar hacks (though this really should
eventually be fixed at a lower level). The wrong gettext in this case is
getting pulled in via:
csound(host) -> fluidsynth(host) -> glib(host) -> gettext(host)

pkgs/applications/audio/csound/default.nix

@@ -31,7 +31,9 @@ stdenv.mkDerivation rec {
   };
 
   cmakeFlags = [ "-DBUILD_CSOUND_AC=0" ] # fails to find Score.hpp
-    ++ lib.optional stdenv.isDarwin "-DCS_FRAMEWORK_DEST=${placeholder "out"}/lib";
+    ++ lib.optional stdenv.isDarwin "-DCS_FRAMEWORK_DEST=${placeholder "out"}/lib"
+    # Ignore gettext in CMAKE_PREFIX_PATH on cross to prevent find_program picking up the wrong gettext
+    ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) "-DCMAKE_IGNORE_PATH=${lib.getBin gettext}/bin";
 
   nativeBuildInputs = [ cmake flex bison gettext ];
   buildInputs = [ libsndfile libsamplerate boost ]