configure.ac + autogen.sh updates

configure.ac:
- remove AM_C_PROTOTYPES: no longer present in automake 1.12,
pre-ANSI compilers are no longer an issue today
(build error with automake 1.12)
- remove AC_C_CONST: deprecated, not really needed
(all compilers available today support const)
- remove AC_ISC_POSIX: deprecated, the only system the needed it
(INTERACTIVE Systems Corporation Unix) is unsupported by it's
vendor for 6 years

autogen.sh:
- update the autoconf and libtool requirements to the ones in configure.ac