tccpp.c: fix GNU comma handling

This requires moving TOK_PLCHLDR handling, but the new logic should make
things easier even if (when?) GNU comma handling is removed.

(Somewhat confusingly, GCC no longer supports GNU commas. See
http://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html for a description
of past and current GCC behaviour.)

tcc.h

@@ -880,6 +880,7 @@ struct TCCState {
 #define TOK_DOTS     0xcc /* three dots */
 #define TOK_SHR      0xcd /* unsigned shift right */
 #define TOK_NOSUBST  0xcf /* means following token has already been pp'd */
+#define TOK_GNUCOMMA 0xd0 /* ,## preprocessing token */
 
 #define TOK_SHL   0x01 /* shift left */
 #define TOK_SAR   0x02 /* signed shift right */