I had some trouble with implementing PPP and PPPOS in LWIP V2.0.3 with a C++
compiler due to these header files not having the #ifdef __cplusplus linkage.
I received a somewhat cryptic error message from my compiler and I also
thought that the whole lwIP library had this linkage because I saw it several
other header files. If possible please add this to these header files it
should help other people save some time in the future.