libtoxcore: repair paths in pkg-config file

pkgs/development/libraries/libtoxcore/default.nix

@@ -33,6 +33,12 @@ let
     doCheck = false; # hangs, tries to access the net?
     checkInputs = [ check ];
 
+    postFixup =''
+      sed -i $out/lib/pkgconfig/*.pc \
+        -e "s|^libdir=.*|libdir=$out/lib|" \
+        -e "s|^includedir=.*|includedir=$out/include|"
+    '';
+
     meta = with stdenv.lib; {
       description = "P2P FOSS instant messaging application aimed to replace Skype";
       homepage = https://tox.chat;