Rework autoconf stuff.

Rework autoconf stuff.

Update to autoconf 2.64.

Use custom macro for ethtool, netsnmp and argument handling to shorten
main configure.ac file.

Use less verbose build process by default.

For headers, missing header is fatal for now because we don't know how
to replace them. Better handling will be done later. We only target
Linux for now with GNU LibC: portability issues are narrow.

Display results at the end of configure process.

AC_C_CONST is obsolete

See:
 http://www.nabble.com/fix-AC_C_CONST-to-work-with-CFLAGS--O2--Wall--Werror-td17164627.html
 http://www.gnu.org/software/autoconf/manual/autoconf.html#index-AC_005fC_005fCONST-790

Some useful documentation about autoconf:
 http://sources.redhat.com/autobook/ (a bit outdated)
 http://www.freesoftwaremagazine.com/books/autotools_a_guide_to_autoconf_automake_libtool
 http://www.flameeyes.eu/autotools-mythbuster/