Print a nicer error message when asked to build an empty string ('').

This happens sometimes, for example, if you do
	whatever | while read x; do
		redo-ifchange "$x"
	done
and the input contains blank lines.

We could ignore the request for blankness, but it seems like that
situation might indicate a more serious bug in your parser, so it's
probably better to just abort with a meaningful error.

builder.py

@@ -282,6 +282,10 @@ def main(targets, shouldbuildfunc):
     seen = {}
     lock = None
     for t in targets:
+        if not t:
+            err('cannot build the empty target ("").\n')
+            retcode[0] = 204
+            break
         assert(state.is_flushed())
         if t in seen:
             continue