* configure.in: Change AC_PREREQ to 2.13. Remove AM_CYGWIN32.

	Change AM_EXEEXT to AC_EXEEXT and AM_PROG_INSTALL to
	AC_PROG_INSTALL.  Add comments to uses of AC_DEFINE.
	* acinclude.m4: Add comments to uses of AC_DEFINE.
	* acconfig.h: Remove.
	* configure: Rebuild with current autoconf/automake.
	* aclocal.m4: Likewise.
	* config.in: Likewise.
	* Makefile.in: Likewise.