Add sup expression
Compile libusb with gccApple on darwin
Add gpgme build input for sup
sup: separate dependencies

create separate expressions for gems requiring native extensions to be
built:

* gpgme
* ncursesw_sup
* xapian_full_alaveteli
Rename ruby gpgme gem to ruby-gpgme
gnupg: use libiconvOrEmpty instead of libiconv (not required for glibc platforms)
sup: use libiconvOrEmpty instead of plain old libiconv
Merge pull request #610 from lovek323/sup

Add sup expression