Fix build failure introduced by previous commit

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40026 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -397,7 +397,7 @@ AS_CASE(["$host_os:$build_os"],
     # Apple clang version 2.0 (tags/Apple/clang-137) (based on LLVM 2.9svn)
     # Apple clang version 2.1 (tags/Apple/clang-163.7.1) (based on LLVM 3.0svn)
     if [ $CC = "clang" ] && $CC --version | grep 'clang version [12 ]\.' ; then
-        AC_MSG_ERROR(clang version 3.0 or later is required)])
+        AC_MSG_ERROR([clang version 3.0 or later is required])
     fi
 ])
 if test x"${build}" != x"${host}"; then