Use dir_config() to set search path for ncurses header

[do not merge yet]

NCURSES_OPAQUE is not set on Mac OS X, dir_config seems to fix this
on the Macports version.

https://trac.macports.org/browser/trunk/dports/ruby/rb-ncurses-ruby/files/extconf.rb.diff

Untested, but a more correct way to fix https://github.com/sup-heliotrope/sup/issues/49.