celt-0.5.1: disable tests that fail to build due to bad code

These were fixed upstream, but patches don't apply to
0.5.1 which we need to use for spice.

Instead of maintaining backported patches for very deprecated
software, just don't build the tests in the first place.

pkgs/development/libraries/celt/0.5.1.nix

···
     url = "http://downloads.xiph.org/releases/celt/celt-${version}.tar.gz";
     sha256 = "0bkam9z5vnrxpbxkkh9kw6yzjka9di56h11iijikdd1f71l5nbpw";
   };
+
+  # Don't build tests due to badness with ec_ilog
+  prePatch = ''
+    substituteInPlace Makefile.in \
+      --replace 'SUBDIRS = libcelt tests' \
+                'SUBDIRS = libcelt'
+  '';
 })

pkgs/development/libraries/celt/generic.nix

···
 { stdenv, version, src
 , liboggSupport ? true, libogg ? null # if disabled only the library will be built
+, prePatch ? ""
 , ...
 }:
 
···
   name = "celt-${version}";
 
   inherit src;
+
+  inherit prePatch;
 
   buildInputs = []
     ++ stdenv.lib.optional liboggSupport libogg;