redo-if{change,create}: print a useful message if REDO isn't set.

Again, I forgot to make vars.py not crash if the variables aren't set, so we
can print a useful error message.  But this time I have the right solution:
vars.py will do the checking for itself, and abort with a nice message.

redo-ifchange.py

@@ -45,10 +45,6 @@ def should_build(t):
     return not state.isbuilt(t) and dirty_deps(t, depth = '')
 
 
-if not vars.TARGET:
-    err('redo-ifchange: error: must be run from inside a .do\n')
-    sys.exit(100)
-
 rv = 202
 try:
     try:

redo-ifcreate.py

@@ -4,10 +4,6 @@ import vars, state
 from helpers import err, mkdirp
 
 
-if not vars.TARGET:
-    err('redo-ifcreate: error: must be run from inside a .do\n')
-    sys.exit(100)
-
 try:
     for t in sys.argv[1:]:
         if os.path.exists(t):

vars.py

@@ -1,6 +1,12 @@
 import os
 import atoi
 
+if not os.environ.get('REDO'):
+    import sys
+    sys.stderr.write('%s: error: must be run from inside a .do\n'
+                     % sys.argv[0])
+    sys.exit(100)
+
 PWD = os.environ.get('REDO_PWD', '')
 TARGET = os.environ.get('REDO_TARGET', '')
 DEPTH = os.environ.get('REDO_DEPTH', '')