cmd/cc: Fix -I switch to handle a path with blankspaces correctly

Currently, -I switch can't deal with a path containing spaces.
This commit simplify setinclude(), by removing the special case
of a string that had spaces. After this change, setinclude() will
merely add the given directories to the include path, if it does
not yet exist, and this approach works.

Will be needed for solving issue 115.

R=agl1, rsc, iant2, r
https://golang.org/cl/155059

src/cmd/cc/lex.c

@@ -1517,16 +1517,11 @@ void
 setinclude(char *p)
 {
 	int i;
-	char *e;
-
-	while(*p != 0) {
-		e = strchr(p, ' ');
-		if(e != 0)
-			*e = '\0';
 
+	if(*p != 0) {
 		for(i=1; i < ninclude; i++)
 			if(strcmp(p, include[i]) == 0)
-				break;
+				return;
 
 		if(i >= ninclude)
 			include[ninclude++] = p;
@@ -1536,9 +1531,6 @@ setinclude(char *p)
 			exits("ninclude");
 		}
 
-		if(e == 0)
-			break;
-		p = e+1;
 	}
 }