brew install -v hg --use-llvm

gistfile1.txt

[Jake@Jake’s MacBook Pro:~]⋙ brew install -v hg --use-llvm
==> Downloading http://mercurial.selenic.com/release/mercurial-2.3.tar.gz
Already downloaded: /Library/Caches/Homebrew/mercurial-2.3.tar.gz
/usr/bin/tar xf /Library/Caches/Homebrew/mercurial-2.3.tar.gz
==> make PREFIX=/usr/local/Cellar/mercurial/2.3 build
make PREFIX=/usr/local/Cellar/mercurial/2.3 build
python setup.py  build
running build
running build_mo
warning: hgbuildmo: could not find msgfmt executable, no translations will be built

running build_ext
building 'mercurial.base85' extension
creating build
creating build/temp.macosx-10.8-i386-2.7
creating build/temp.macosx-10.8-i386-2.7/mercurial
/usr/bin/llvm-gcc -DNDEBUG -g -Os -Wall -Wstrict-prototypes -Os -w -pipe -isystem /usr/local/include –arch x86_64 -pipe -I/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c mercurial/base85.c -o build/temp.macosx-10.8-i386-2.7/mercurial/base85.o
i686-apple-darwin11-llvm-gcc-4.2: –arch: No such file or directory
i686-apple-darwin11-llvm-gcc-4.2: x86_64: No such file or directory
error: command '/usr/bin/llvm-gcc' failed with exit status 1
make: *** [build] Error 1
==> Exit Status: 2
https://github.com/mxcl/master/blob/master/Library/Formula/mercurial.rb#L30
==> Build Environment
HOMEBREW_VERSION: 0.9.2
HEAD: 8458b4092bba013853c088bb8cad9d4aa8dd02b0
CPU: quad-core 64-bit arrandale
OS X: 10.8-x86_64
Xcode: 4.4.1
CLT: 4.4.0.0.1.1249367152
X11: 2.7.2 in /opt/X11
"--use-llvm" was specified
CC: /usr/bin/llvm-gcc => /usr/llvm-gcc-4.2/bin/llvm-gcc-4.2
CXX: /usr/bin/llvm-g++ => /usr/llvm-gcc-4.2/bin/llvm-g++-4.2
LD: /usr/bin/llvm-gcc => /usr/llvm-gcc-4.2/bin/llvm-gcc-4.2
CFLAGS: -Os -w -pipe
CXXFLAGS: -Os -w -pipe
CPPFLAGS: -isystem /usr/local/include
LDFLAGS: -L/usr/local/lib
MAKEFLAGS: -j4
MACOSX_DEPLOYMENT_TARGET: 10.8
PKG_CONFIG_PATH: /usr/local/Library/Homebrew/pkgconfig
OBJC: /usr/bin/llvm-gcc
PATH: /usr/local/bin:/usr/local:/usr/local/sbin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/local/Library/Contributions/cmds

Error: Failed executing: make PREFIX=/usr/local/Cellar/mercurial/2.3 build (mercurial.rb:30)

These existing issues may help you:
    https://github.com/mxcl/homebrew/issues/14226
Otherwise, this may help you fix or report the issue:
    https://github.com/mxcl/homebrew/wiki/bug-fixing-checklist

我也遇到这个问题了