tccpp: Fix token pasting

See testcase.  We must always paste tokens (at least if not
currently substing a normal argument, which is a speed optimization
only now) but at the same time must not regard a ## token
coming from argument expansion as the token-paste operator, nor
if we constructed a ## token due to pasting itself (that was already
checked by pp/01.c).

tcc.h

@@ -899,6 +899,7 @@ struct filespec {
 #define TOK_TWOSHARPS 0xca /* ## preprocessing token */
 #define TOK_PLCHLDR  0xcb /* placeholder token as defined in C99 */
 #define TOK_NOSUBST  0xcc /* means following token has already been pp'd */
+#define TOK_PPJOIN   0xce /* A '##' in the right position to mean pasting */
 
 #define TOK_SHL   0x01 /* shift left */
 #define TOK_SAR   0x02 /* signed shift right */