configure: make AM_SILENT_RULES conditional

Signed-off-by: Davide Madrisan <davide.madrisan@gmail.com>
configure: avoid 'AS_CASE' for compatibility

Signed-off-by: Davide Madrisan <davide.madrisan@gmail.com>
configure: do not use 'AC_TYPE_SSIZE_T' as it requires autoconf 2.60+

Signed-off-by: Davide Madrisan <davide.madrisan@gmail.com>
Fix the "implicit definition of 'getline'" warnings by adding the required '#define _GNU_SOURCE'

Signed-off-by: Davide Madrisan <davide.madrisan@gmail.com>
Fix the build for systems without 'linux/if_link.h'

Signed-off-by: Davide Madrisan <davide.madrisan@gmail.com>