Update build process to automake-1.13

In the 1.13 version of automake, a lot of deprecated commands were removed and PNMixer doesn't compile anymore.
- Use AC_CONFIG_HEADER instead of AM_CONFIG_HEADER
- Move parameters for AM_INIT_AUTOMAKE to AC_INIT
- Remove AM_PROG_CC_STDC as AC_PROG_CC is sufficient
- Move configure.in to configure.ac
- Use AM_CFLAGS instead of INCLUDES as the later is no longer included
- Remove autogen.sh in favor of autoreconf