Fix tool script on Linux with missing pkg-config

I had believed this case was working correctly, but an edit to reduce
the number of used local variables had introduced a mistake that caused
it to fail. This commit fixes the problem by overwriting the variable
where the captured output of a failed pkg-config attempt was stored with
0, so that the subsequent if branch works correctly.

tool

@@ -419,6 +419,8 @@ build_target() {
           # split by spaces into separate args, then append to array
           IFS=" " read -r -a libraries <<< "$curses_flags"
           curses_flags=1
+        else
+          curses_flags=0
         fi
       fi
       # If we didn't get the flags by pkg-config, just guess. (This will work