Do not evaluate DEF statements inside of IF blocks with false conditions.

Closes #1796

Cython/Compiler/Parsing.py

@@ -2160,6 +2160,7 @@ def p_DEF_statement(s):
     name = p_ident(s)
     s.expect('=')
     expr = p_compile_time_expr(s)
+    if s.compile_time_eval:
         value = expr.compile_time_value(denv)
         #print "p_DEF_statement: %s = %r" % (name, value) ###
         denv.declare(name, value)

tests/run/ct_IF.pyx

@@ -42,3 +42,33 @@ def h():
     ELSE:
         i = 3
     return i
+
+
+def control_flow_DEF1():
+    """
+    >>> control_flow_DEF1()
+    B should be 2.
+    2
+    """
+    IF YES:
+        DEF B=2
+        print('B should be 2.')
+    ELSE:
+        DEF B=3
+        print('B should be 3.')
+    return B
+
+
+def control_flow_DEF2():
+    """
+    >>> control_flow_DEF2()
+    B should be 3.
+    3
+    """
+    IF NO:
+        DEF B=2
+        print('B should be 2.')
+    ELSE:
+        DEF B=3
+        print('B should be 3.')
+    return B