Call init_mkmf for main

When MakeMakefile is extended on itself there is a call to init_mkmf.
This method mainly setups various global variables, but it also sets @libdir_basename to default value "lib".

When MakeMakefile is included in main object there is no call to init_mkmf.
That's why @libdir_basename is not set for main object.

Therefore when we invoke mkmf methods on main object @libdir_basename is nil.
That leads to incorrect lib path when --with-XXX-dir is used.