Don't force recompile with the new flag

theano/configdefaults.py

@@ -1153,7 +1153,9 @@ AddConfigVar('cmodule.age_thresh_use',
 AddConfigVar('cmodule.debug',
              "If True, define a DEBUG macro (if not exists) for any compiled C code.",
              BoolParam(False),
-             in_c_key=True)
+             # Do not add it in the c key when we keep use the old default.
+             # To do not recompile for no good reason.
+             in_c_key=lambda: theano.config.cmodule.debug)
 
 
 def default_blas_ldflags():

theano/configparser.py

@@ -191,7 +191,16 @@ def get_config_md5():
 
     We only take into account config options for which `in_c_key` is True.
     """
-    all_opts = sorted([c for c in _config_var_list if c.in_c_key],
+    all_opts = []
+    for c in _config_var_list:
+        if callable(c.in_c_key):
+            i = c.in_c_key()
+        else:
+            i = c.in_c_key
+        if i:
+            all_opts.append(c)
+
+    all_opts = sorted(all_opts,
                       key=lambda cv: cv.fullname)
     return theano.gof.utils.hash_from_code('\n'.join(
         ['%s = %s' % (cv.fullname, cv.__get__(True, None)) for cv in all_opts]))