Fix a regression bug
Fix a regression bug

GitHub: fix GH-196

It's caused when name that includes "/" is specified to
Rake::ExtensionTask.new and the default lib_dir is used such as
https://github.com/konsolebox/digest-xxhash-ruby . In the case, built
binaries are copied to lib/ not lib/XXX/.

Reported by konsolebox. Thanks!!!

rake-compiler/rake-compiler