gem-config: add ethon

The Ethon gem uses libcurl.so at runtime via FFI, so we need to specify the correct location.

pkgs/development/ruby-modules/gem-config/default.nix

@@ -87,6 +87,14 @@ in
       sed -i $installPath/lib/dep-selector-libgecode.rb -e 's@VENDORED_GECODE_DIR =.*@VENDORED_GECODE_DIR = "${gecode_3}"@'
     '';
   };
+  
+  ethon = attrs: {
+    dontBuild = false;
+    postPatch = ''
+      substituteInPlace lib/ethon/curls/settings.rb \
+        --replace "libcurl" "${curl.out}/lib/libcurl.so"
+    '';
+  }; 
 
   eventmachine = attrs: {
     buildInputs = [ openssl ];