muniversal PG could use $prefix/bin/lipo and $prefix/bin/libtool if available

[RJVB (René Bertin)]

Using macports-clang versions newer than the host's AppleClang (clang from Xcode) you can end up with binaries that the host's lipo (and presumably libtool) command won't know how to handle. Example: static libraries from -flto=thin builds with clang-mp-5+ on OS X 10.9 .

The lipo command from port:cctools has no problems with those files.

You'd probably want to use the version from port:cctools only when a macports clang compiler is used, that should prevent reproducible build violations.