use pkg-config for ncursesw directories

[ymmtmdk]

Using pkg-config for searching ncursesw directories. This request may solves issues like this:
#17

[gauteh]

Hi, we are using https://github.com/terceiro/rmail now. It should hopefully be more maintained.

[ymmtmdk]

Sorry for my poor explanation.
I want to use just ncursesw only.
But gem install ncursesw shows the build error on my Mac and I want to solve that.
This patch will resolve build problems on Mac by adding pkg-config to the search method of Homebrewed ncurses.

[gauteh]

err. yeah I am mixing things up. sorry.

[ymmtmdk]

Thank you!