% brew install tig
checking for iconv... no, consider installing GNU libiconv
configure: error: in `/private/tmp/brew-tig-1.0-pinS/tig-1.0':
configure: error: iconv() not found. Please install libiconv and use --with-libiconv=/path/to/dir.


/usr/libにはlibiconvが入っていたので、brew install tig --with-libconv=/usr/lib/ としても効果無し。んで本家のissuesを眺めていたところ、

iconv() not found error when brew install tig · Issue #1817 · mxcl/homebrew

brew doctorの結果見てみて〜とのコメント発見。実際に実行してみると、

% brew doctor
Warning: libiconv files detected at a system prefix other than /usr
Homebrew doesn't provide a libiconv formula, and expects to link against
the system version in /usr. libiconv in other prefixes can cause
compile or link failure, especially if compiled with improper
architectures. OS X itself never installs anything to /usr/local so
it was either installed by a user or some other third party software.

tl;dr: delete these files:

どうやら何かのソフトウェアを入れた際に /usr/local/include/ にiconvヘッダが置かれたようで、削除したらうまくインストールできました。このエラーはdoctorが無いと気づけなかったな・・