Improved error message when using llvm-gcc

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@33780 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -293,7 +293,7 @@ if test "$GCC" = yes; then
       @%:@if defined __llvm__ && !defined __clang__
       @%:@error llvm-gcc has ended
       @%:@endif
-    ], [], [], [AC_MSG_ERROR(llvm-gcc has ended)])
+    ], [], [], [AC_MSG_ERROR(llvm-gcc has ended. Run configure with CC=clang)])
     linker_flag=-Wl,
     : ${optflags=-O3}
     AS_CASE(["$target_os"], [linux*|darwin*], [: ${debugflags=-ggdb}])