bugfix: include config.h before any other headers

For some reason, flex used to prepend several standard headers before
config.h. This prevented some conditional extension in these headers
from being defined.

The explicit prototype for strdup() shouldn't be required anymore as
it should be provided by string.h.
configure: remove hardcoded compiler mode

Specifying the mode explicitly shouldn't be necessary as the
AC_PROG_CC_C99 autoconf macro should determine a compatible mode
itself.
configure: forbid implicitly declared functions
configure: replace deprecated autoconf macro

The new macro depends on autoconf >= 2.60.
The AllowedSender directive misbehaved if network was down during startup

The passing of the AI_ADDRCONFIG flag to getaddrinfo() caused IP
prefixes to be interpreted as hostname wildcards if network was down
during rsyslog's startup.
pstats: add "origin" paramter to stats records

see also: https://github.com/rsyslog/rsyslog/issues/119