Update the build file tracker example

playground/track_files_timestamps/wscript

@@ -12,7 +12,7 @@ from waflib import Logs, Options
 
 def options(opt):
 	opt.load('compiler_c')
-	opt.load('build_file_tracker')
+	opt.load('build_file_timestamp')
 
 def configure(conf):
 	conf.load('compiler_c')

waflib/extras/build_file_timestamp.py

@@ -3,12 +3,11 @@
 # Thomas Nagy, 2015
 
 """
-Force files to depend on the timestamps of those located in the build directory. You may
+Force files to also depend on the timestamps of those located in the build directory. You may
 want to use this to force partial rebuilds, see playground/track_output_files/ for a working example.
 
 Note that there is a variety of ways to implement this, one may want use timestamps on source files too for example,
-or one may want to hash the files in the source directory only under certain conditions (md5_tstamp tool)
-or to hash the file in the build directory with its timestamp
+or one may want to hash theA files in the source directory only under certain conditions (md5_tstamp tool)
 """
 
 import os
@@ -18,11 +17,8 @@ def get_bld_sig(self):
 	if not self.is_bld() or self.ctx.bldnode is self.ctx.srcnode:
 		return Utils.h_file(self.abspath())
 
-	try:
-		# add the creation time to the signature
-		return self.sig + str(os.stat(self.abspath()).st_mtime)
-	except AttributeError:
-		return None
+	val = Utils.h_file(self.abspath()) + str(os.stat(self.abspath()).st_mtime).encode('latin-1')
+	return val
 
 Node.Node.get_bld_sig = get_bld_sig