Tested using the pure-darwin alternate stdenv on Yosemite.
I need this change for gnupg to build. Otherwise, I get this error:
```
configure:
***
*** The system does not provide a working iconv function. Please
*** install a suitable library; for example GNU Libiconv which is
*** available at:
*** http://ftp.gnu.org/gnu/libiconv/
***
configure: error:
***
*** Required libraries not found. Please consult the above messages
*** and install them before running configure again.
***
builder for ‘/nix/store/pvzqmfzc05ifmvi0vglyl2xlkcdgh6c2-gnupg-2.1.6.drv’ failed with exit code 1
```
cc: @wkennington