Compiler directive: warning_undeclared

[dagss]

A directive to warn/give error on any undeclared variables? I.e.:

@cython.warning_undeclared(True)
def foo():
    global g
    cdef object a
    cdef int b
    g = 3 # ok
    a = 3 # ok
    b = 3 # ok
    c = 3 # warns that "c" is not declared

error_undeclared should probably be available as well.

Implementation note:

In this specific case I think I'd tend to add a few lines in 
analyse_target_type (or whatever) in NameNode I think, without using a transform (where the directives in effect are available in 
env.directives or similar

[robertwb]

Ideally, this should be rolled into a directive like

@cython.warn(undeclared=True)

[robertwb]

Depends on #479.