brew install git

gistfile1.txt

==> Downloading http://git-core.googlecode.com/files/git-1.7.9.1.tar.gz
File already downloaded in /Users/rory/Library/Caches/Homebrew
/usr/bin/tar xf /Users/rory/Library/Caches/Homebrew/git-1.7.9.1.tar.gz
==> make prefix=/usr/local/Cellar/git/1.7.9.1 CC=/Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc CFLAGS=-Os -w -pipe -march=core2 -msse4.1 LDFLAGS= install
make prefix=/usr/local/Cellar/git/1.7.9.1 CC=/Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc CFLAGS=-Os -w -pipe -march=core2 -msse4.1 LDFLAGS= install
GIT_VERSION = 1.7.9.1
    * new build flags or prefix
    * new link flags
./generate-cmdlist.sh > common-cmds.h+ && mv common-cmds.h+ common-cmds.h
/Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc -o hex.o -c -MF ./.depend/hex.o.d -MMD -MP  -Os -w -pipe -march=core2 -msse4.1 -I. -DUSE_ST_TIMESPEC -DNO_GETTEXT  -DHAVE_DEV_TTY -DSHA1_HEADER='<openssl/sha.h>'  -DNO_MEMMEM  hex.c
In file included from git-compat-util.h:239,
                 from cache.h:4,
                 from hex.c:1:
compat/bswap.h:12: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘default_swab32’
compat/bswap.h:25: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘git_bswap32’
In file included from cache.h:4,
                 from hex.c:1:
git-compat-util.h:319: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘read_in_full’
git-compat-util.h:436: error: expected declaration specifiers or ‘...’ before ‘off_t’
git-compat-util.h:437: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘xread’
git-compat-util.h:438: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘xwrite’
git-compat-util.h:440: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘*’ token
git-compat-util.h:446: error: expected ‘)’ before ‘len’
git-compat-util.h: In function ‘strtoul_ui’:
git-compat-util.h:499: error: ‘errno’ undeclared (first use in this function)
git-compat-util.h:499: error: (Each undeclared identifier is reported only once
git-compat-util.h:499: error: for each function it appears in.)
git-compat-util.h: In function ‘strtol_i’:
git-compat-util.h:512: error: ‘errno’ undeclared (first use in this function)
In file included from cache.h:5,
                 from hex.c:1:
strbuf.h: At top level:
strbuf.h:111: error: expected declaration specifiers or ‘...’ before ‘FILE’
strbuf.h:113: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘strbuf_read’
strbuf.h:117: error: expected declaration specifiers or ‘...’ before ‘FILE’
strbuf.h:118: error: expected declaration specifiers or ‘...’ before ‘FILE’
In file included from hex.c:1:
cache.h:21: error: expected specifier-qualifier-list before ‘z_stream’
cache.h: In function ‘create_ce_mode’:
cache.h:268: error: ‘S_IFLNK’ undeclared (first use in this function)
cache.h:269: error: ‘S_IFMT’ undeclared (first use in this function)
cache.h:271: error: ‘S_IFREG’ undeclared (first use in this function)
cache.h: In function ‘ce_to_dtype’:
cache.h:291: error: ‘S_IFMT’ undeclared (first use in this function)
cache.h: In function ‘canon_mode’:
cache.h:301: error: ‘S_IFREG’ undeclared (first use in this function)
cache.h:303: error: ‘S_IFLNK’ undeclared (first use in this function)
cache.h:305: error: ‘S_IFDIR’ undeclared (first use in this function)
cache.h: In function ‘object_type’:
cache.h:393: error: ‘S_IFMT’ undeclared (first use in this function)
cache.h: At top level:
cache.h:559: error: expected specifier-qualifier-list before ‘pid_t’
cache.h:775: error: expected declaration specifiers or ‘...’ before ‘time_t’
cache.h:969: error: expected specifier-qualifier-list before ‘off_t’
cache.h:978: error: expected specifier-qualifier-list before ‘off_t’
cache.h:996: error: expected specifier-qualifier-list before ‘off_t’
cache.h:1054: error: expected declaration specifiers or ‘...’ before ‘off_t’
cache.h:1060: error: expected declaration specifiers or ‘...’ before ‘uint32_t’
cache.h:1061: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘nth_packed_object_offset’
cache.h:1062: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘find_pack_entry_one’
cache.h:1064: error: expected declaration specifiers or ‘...’ before ‘off_t’
cache.h:1066: error: expected declaration specifiers or ‘...’ before ‘off_t’
cache.h:1067: error: expected declaration specifiers or ‘...’ before ‘off_t’
cache.h:1091: error: expected specifier-qualifier-list before ‘off_t’
cache.h:1158: error: expected ‘)’ before ‘*’ token
cache.h:1167: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘read_in_full’
cache.h:1168: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘write_in_full’
cache.h:1169: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘write_str_in_full’
cache.h:1240: error: expected declaration specifiers or ‘...’ before ‘FILE’
make: *** [hex.o] Error 1
make: *** Waiting for unfinished jobs....
==> Exit Status: 2
http://github.com/mxcl/homebrew/blob/master/Library/Formula/git.rb#L47
==> Environment
HOMEBREW_VERSION: 0.8.1
HEAD: (none)
HOMEBREW_PREFIX: /usr/local
HOMEBREW_CELLAR: /usr/local/Cellar
Hardware: dual-core 64-bit penryn
OS X: 10.7.3
Kernel Architecture: x86_64
Xcode: 4.2
GCC-4.0: N/A
GCC-4.2: N/A
LLVM: build 2336
Clang: N/A
MacPorts or Fink? false
X11 installed? true
System Ruby: 1.8.7-249
/usr/bin/ruby => /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
Which Perl:   /usr/bin/perl
Which Python: /usr/bin/python
Which Ruby:   /usr/bin/ruby => /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
==> Build Flags
CC: /Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc => /Applications/Xcode.app/Contents/Developer/usr/llvm-gcc-4.2/bin/llvm-gcc-4.2
CXX: /Applications/Xcode.app/Contents/Developer/usr/bin/llvm-g++ => /Applications/Xcode.app/Contents/Developer/usr/llvm-gcc-4.2/bin/llvm-g++-4.2
LD: /Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc => /Applications/Xcode.app/Contents/Developer/usr/llvm-gcc-4.2/bin/llvm-gcc-4.2
CFLAGS: -Os -w -pipe -march=core2 -msse4.1
CXXFLAGS: -Os -w -pipe -march=core2 -msse4.1
MAKEFLAGS: -j2

Error: Failed executing: make prefix=/usr/local/Cellar/git/1.7.9.1 CC=/Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc CFLAGS=-Os\ -w\ -pipe\ -march=core2\ -msse4.1 LDFLAGS= install
These existing issues may help you:
    https://github.com/mxcl/homebrew/issues/8977
    https://github.com/mxcl/homebrew/issues/9023
    https://github.com/mxcl/homebrew/issues/9618
    https://github.com/mxcl/homebrew/issues/10081
    https://github.com/mxcl/homebrew/issues/10225
Otherwise, please report the bug:
    https://github.com/mxcl/homebrew/wiki/checklist-before-filing-a-new-issue

Hey Rory. Have you looked into this further yet? I'm curious as to what the cause was for you?

Matt
www.notch8.com

It was in reference to this issue here Homebrew/legacy-homebrew#10244 which seems to have been resolved.

Perfect, thanks I ran into the same issue while migrating to a new machine.