rp-pppoe: add systemd support and fix error in the step of do_configure.

The error is as follows:
error: unrecognized command line option '-V'
conftest.c:9:28: fatal error: ac_nonexistent.h:
No such file or directory #include <ac_nonexistent.h>.

Signed-off-by: Li Xin <lixin.fnst@cn.fujitsu.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>