gfortran brew failed

$ brew doctor
Warning: Your Homebrew is not installed to /usr/local
You can install Homebrew anywhere you want, but some brews may only build
correctly if you install in /usr/local. Sorry!

$ brew install gfortran
Warning: Building source; cellar of gfortran's bottle is /usr/local/Cellar
==> Downloading http://ftpmirror.gnu.org/gcc/gcc-4.8.1/gcc-4.8.1.tar.bz2
Already downloaded: /opt/boxen/cache/homebrew/gfortran-4.8.1.tar.bz2
==> ../configure --prefix=/opt/boxen/homebrew/Cellar/gfortran/4.8.1/gfortran --datarootdir=/opt/boxen/homebrew/Cellar/gfortran/4.8.1/share --bindir=/opt/boxen/homebrew/Cellar/gfortran/4.8.1/bin --enabl
==> make bootstrap
libtool: link: ( cd ".libs" && rm -f "libitm.la" && ln -s "../libitm.la" "libitm.la" )
no
configure: error: GNU Fortran is not working; please report a bug in http://gcc.gnu.org/bugzilla, attaching /private/tmp/gfortran-DHKp/gcc-4.8.1/build/x86_64-apple-darwin12.5.0/libgfortran/config.log
make[1]: *** [configure-target-libgfortran] Error 1
make: *** [bootstrap] Error 2

$ brew --config                                                                                                                                                                        
HOMEBREW_VERSION: 0.9.5
ORIGIN: https://github.com/mxcl/homebrew.git
HEAD: 96c855c004e55d227cbe3e4ef716d6a916219c2a
HOMEBREW_PREFIX: /opt/boxen/homebrew
HOMEBREW_CELLAR: /opt/boxen/homebrew/Cellar
CPU: dual-core 64-bit penryn
OS X: 10.8.5-x86_64
Xcode: 5.0 => /Applications/Xcode5-DP2.app/Contents/Developer
CLT: 5.0.0.0.1.1376620630
GCC-4.2: build 5666
LLVM-GCC: build 2336
Clang: 5.0 build 500
X11: 2.7.4 => /opt/X11
System Ruby: 1.8.7-358
Perl: /usr/bin/perl
Python: /usr/bin/python
Ruby: /opt/boxen/rbenv/shims/ruby