openexrid-unstable: fix clang build

/cc ZHF #36454

pkgs/development/libraries/openexrid-unstable/default.nix

@@ -16,6 +16,11 @@ stdenv.mkDerivation rec
 
   patches = [ ./openexrid.patch ];
 
+  postPatch = ''
+    substituteInPlace openexrid/makefile \
+        --replace g++ c++
+  '';
+
   NIX_CFLAGS_COMPILE=''-I${ilmbase.dev}/include/OpenEXR
                        -I${openexr.dev}/include/OpenEXR
                        -I${openfx.dev}/include/OpenFX
@@ -38,7 +43,7 @@ stdenv.mkDerivation rec
       find $out
       mv $out/include $dev/
       mv $out/lib $lib/
-      '';
+  '';
 
   meta = with stdenv.lib; {
     description = "OpenEXR files able to isolate any object of a CG image with a perfect antialiazing";