autoconf/m4/cxx_flag_check.m4 - llvm-project/llvm - Git at Google

 AC_DEFUN([CXX_FLAG_CHECK],
   [AC_SUBST($1, `$CXX -Werror patsubst($2, [^-Wno-], [-W]) -fsyntax-only -xc /dev/null 2>/dev/null && echo $2`)])
	AC_DEFUN([CXX_FLAG_CHECK],
	[AC_SUBST($1, `$CXX -Werror patsubst($2, [^-Wno-], [-W]) -fsyntax-only -xc /dev/null 2>/dev/null && echo $2`)])