add clang support along with gcc

[vjardin]

Since clang is pretty aggressive, it allows to detect some weird cases such as:

gtp_ppp.c:1994:14: warning: implicit conversion from 'int' to 'char' changes value from 129 to -127 [-Wconstant-conversion]
                opt[i++] = IPCP_OPT_PRIMDNS;
                         ~ ^~~~~~~~~~~~~~~~
include/gtp_ppp.h:65:26: note: expanded from macro 'IPCP_OPT_PRIMDNS'
#define IPCP_OPT_PRIMDNS        129     /* primary remote dns address */
                                ^~~

TODO: if this pull request is accepted, I will (1) try to fix the issues raised by clang and then (2) extend the compilation tests to run with both gcc and clang.