configure.ac: Intentionally underquote m4_normalize calls

This works around a bug in Autoconf 2.64 and earlier, which generated
ridiculous symbols like
  /* #undef HAVE_M4_NORMALIZE___FCNTL_H__LANGINFO_H__LIMITS_H__… */
instead of the ones we need.  It was fixed by v2.65~21:
  http://git.savannah.gnu.org/gitweb/?p=autoconf.git;a=commitdiff;h=81c80d1bf6c8ddf732d0b89f6fc5e306dac9705e

Signed-off-by: Anders Kaseorg <andersk@mit.edu>