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Add support for ## preprocessor operator

It is known that current implementation can have some problems
with macros which have @ or $ characters in the definition, or
in the parameters, but it is not a problem for a C program,
because such characters only can apper between "" or ''.

--- a/cc1/cpp.c

+++ b/cc1/cpp.c

@@ -154,6 +154,13 @@

 		if (c != '@') {

 			if (c == '#')

 				continue;

+			if (c == '$') {

+				while (bp[-1] == ' ')

+					--bp, ++bufsiz;

+				while (s[1] == ' ')

+					++s;

+				continue;

+			}

 			if (bufsiz-- == 0)

 				goto expansion_too_long;

 			*bp++ = c;

@@ -285,6 +292,11 @@

 	size_t len;

 	int prevc = 0, ispar;

+	if (yytoken == '$') {

+		cpperror("'##' cannot appear at either end of a macro expansion");

+		return 0;

+	}

+

 	for (;;) {

 		ispar = 0;

 		if (yytoken == IDEN && nargs >= 0) {

@@ -308,10 +320,15 @@

 			cpperror("too long macro");

 			return 0;

 		}

-		memcpy(bp, yytext, len);

-		bp += len;

-		bufsiz -= len;

-		if ((prevc = yytoken) != '#')

+		if (yytoken == '$') {

+			*bp++ = '$';

+			 --bufsiz;

+		} else {

+			memcpy(bp, yytext, len);

+			bp += len;

+			bufsiz -= len;

+		}

+		if ((prevc  = yytoken) != '#')

 			*bp++ = ' ';

 		next();

 	}

--- a/cc1/lex.c

+++ b/cc1/lex.c

@@ -525,6 +525,7 @@

 	case '*': t = follow('=', MUL_EQ, '*'); break;

 	case '/': t = follow('=', DIV_EQ, '/'); break;

 	case '!': t = follow('=', NE, '!'); break;

+	case '#': t = follow('#', '$', '#'); break;

 	case '-': t = minus(); break;

 	case '+': t = plus(); break;

 	case '.': t = dot(); break;