Produce configure.ac instead of configure.in.

Avoid
    warning: autoconf input should be named 'configure.ac', not 'configure.in'

which would break later:
  http://lists.gnu.org/archive/html/automake/2013-01/msg00000.html

- Automake 1.14 will drop support for the long-deprecated
  'configure.in' name for the Autoconf input file.  You are advised
  to start using the recommended name 'configure.ac' instead, ASAP.
Use AM_CPPFLAGS instead of INCLUDES in Makefile.am.

Avoid
    warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
changelog
Merge pull request #15 from mvidner/automake-warnings

fixed automake warnings about configure.in and INCLUDES