Setting unused -L really makes libtool get angry

Since default Debian/Ubuntu mruby package does not enable C++ ABI
mode, it cannot be used with our C++ code, since mruby exception does
not properly cleanup C++ object allocated on stack.  We have to build
mruby with conf.enable_cxx_abi explicitly.  It is safe for us to build
our own mruby.

This change is required to show path attribute to mruby script.  It is
desirable to construct URI from parts.  Just checking method and path
is "*" is awkward.