mariadb: fix non-Darwin builds after 86dae701

I took care to get the same derivation as before that commit on Linux
and not to change derivation on Darwin.

pkgs/servers/sql/mariadb/default.nix

@@ -93,12 +93,15 @@ stdenv.mkDerivation rec {
     mv $out/lib $lib
     mv $out/include $lib
 
+  ''
+  + stdenv.lib.optionalString stdenv.isDarwin ''
     # Fix library rpaths
     # TODO: put this in the stdenv to prepare for wide usage of multi-output derivations
     for file in $(grep -rl $out/lib $lib); do
       install_name_tool -delete_rpath $out/lib -add_rpath $lib $file
     done
 
+  '' + ''
     # Fix the mysql_config
     sed -i $out/bin/mysql_config \
       -e 's,-lz,-L${zlib}/lib -lz,g' \