gettext: fix darwin build

The CF configure flags broke with the last update but based on
fa9c81f6 and the comment it seems like
these are not necessary anymore.

    $ nix-build -A stdenv.__bootPackages.stdenv.__bootPackages.gettext
    /nix/store/dii9p8717rjnbzar4kh1q3sxk77dbbkd-gettext-0.20.1

    $ nix-store -qR /nix/store/dii9p8717rjnbzar4kh1q3sxk77dbbkd-gettext-0.20.1
    /nix/store/2ws9cmamvr7xyvdg4d2nnd1bmr1zjrrq-bootstrap-tools
    /nix/store/2yn28axagwyifnvdm8npy2f982jxr986-libiconv-osx-10.12.6
    /nix/store/dii9p8717rjnbzar4kh1q3sxk77dbbkd-gettext-0.20.1