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Add #else clause

This implementation needs more testing, and it need a bit of
rewriting, but at this point we have the preprocessor finished.

--- a/cc1/cpp.c

+++ b/cc1/cpp.c

@@ -418,20 +418,19 @@

 	memcpy(yytext, s, len);

 	yytext[len] = '\0';

 	cleanup(endp);

-	curif = numif++;

+	++numif;

-	if (iffalse != 0) {

+	if (iffalse == 0) {

 		sym = lookup(NS_CPP);

 		if ((sym->flags & ISDEFINED) != 0 == isdef)

 			return 1;

 	}

-	++iffalse;

-	while (curif != numif) {

+	curif = iffalse++;

+	while (curif != iffalse) {

 		if (!moreinput())

 			error("found EOF while ...");

 	}

-	--iffalse;

 	return 1;

 }

@@ -451,6 +450,7 @@

 static bool

 endif(char *s)

 {

+	cleanup(s);

 	if (numif == 0)

 		error("#endif without #if");

 	--numif;

@@ -457,6 +457,27 @@

 	return iffalse == 0;

 }

+static bool

+elseclause(char *s)

+{

+	unsigned curif;

+

+	cleanup(s);

+	if (numif == 0)

+		error("#else without #if");

+

+	if (iffalse == 0) {

+		curif = iffalse++;

+		while (curif != iffalse) {

+			if (!moreinput())

+				error("found EOF while ...");

+		}

+	}

+	--iffalse;

+

+	return iffalse != 0;

+}

+

 bool

 preprocessor(char *p)

 {

@@ -471,6 +492,7 @@

 		"ifdef", ifdef,

 		"ifndef", ifndef,

 		"endif", endif,

+		"else", elseclause,

 		"line", line,

 		"pragma", pragma,

 		"error", usererr,