Fix a build problem with clang and --with-opt-dir.

* configure.in: Fix a build problem with clang and --with-opt-dir.
  If ruby is configured with --with-opt-dir=dir when using clang
  as compiler, a warning `clang: warning: argument unused during
  compilation: '-I dir'` is emitted almost every time clang
  compiles a file.  Unfortunately, RUBY_CHECK_PRINTF_PREFIX takes
  any output from the compiler as fatal error, and the check thus
  fails due to the warning.  This is an attempt to fix the problem
  by adding a flag -Qunused-arguments to CFLAGS locally in the
  function to suppress the warning.