autotools junk: Stop using (and checking for) arpa/inet.h

We don't actually need it, and it appears to break the autotools
chain on SLES 11 SP3. Since we only use it in one of the library
test-files, we just remove it and stop checking for it.

Signed-off-by: Andreas Ericsson <ae@op5.se>
autotools: s/INCLUDES/AM_CPPFLAGS/

Apparently this option has been renamed in the autotools machinery
sometime, so at least with 2.69 it produces a warning. Since we
intend to use modern autotools to generate makefiles etc for when
we create releases, we should be just fine with tha patch like this.

Signed-off-by: Andreas Ericsson <ae@op5.se>