rp-pppoe 3.6 autoconf warnings

[Peter Santoro]

FYI,

When building rp-pppoe 3.6 
(http://www.roaringpenguin.com/penguin/open_source_rp-pppoe.php) on 
slackware 10.1 with updates (kernel 2.4.31, autoconf 2.59), I get 
"Present but cannot be compiled" warnings for the following include files:

linux/if_pppox.h
linux/if.h 

Peter