dist: fix Plan 9 build

Since CL 115060044, mkanames declares an empty
array in anames8.c and anames6.c, which is not
valid for the Plan 9 compiler.

char* cnames8[] = {
};

This change makes mkanames not declaring the
cnames array when no C_ constants are found.

LGTM=iant
R=minux, iant
CC=golang-codereviews
https://golang.org/cl/117680043

src/cmd/dist/buildgc.c

@@ -69,7 +69,7 @@ gcopnames(char *dir, char *file)
 void
 mkanames(char *dir, char *file)
 {
-	int i, ch;
+	int i, j, ch;
 	Buf in, b, out, out2;
 	Vec lines;
 	char *p;
@@ -108,6 +108,7 @@ mkanames(char *dir, char *file)
 	}
 	bwritestr(&out, "};\n");
 
+	j=0;
 	bprintf(&out2, "char*	cnames%c[] = {\n", ch);
 	for(i=0; i<lines.len; i++) {
 		if(hasprefix(lines.p[i], "\tC_")) {
@@ -119,9 +120,11 @@ mkanames(char *dir, char *file)
 				*p = '\0';
 			p = lines.p[i] + 3;
 			bwritestr(&out2, bprintf(&b, "\t\"%s\",\n", p));
+			j++;
 		}
 	}
 	bwritestr(&out2, "};\n");
+	if(j>0)
 		bwriteb(&out, &out2);
 
 	writefile(&out, file, 0);