/usr/lib/llvm-3.1/build/autoconf/m4/cxx_flag_check.m4 is in llvm-3.1-dev 3.1-2ubuntu1~12.04.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | AC_DEFUN([CXX_FLAG_CHECK],
[AC_SUBST($1, `$CXX -Werror $2 -fsyntax-only -xc /dev/null 2>/dev/null && echo $2`)])
|