Controlling placement of `cdef extern from` blocks

[Jacob Faibussowitsch US]

I have a function defined in C code (in an `cdef extern from` block) where I want to detect if CYTHON_TRACE is defined. However, it seems that `extern from` blocks are placed above the section where all of the CYTHON_* pre-processor variables are handled.

I then tried to do it in a python function:

```

cdef extern from *:
    int CYTHON_TRACE

cdef void foo():

    if CYTHON_TRACE:

          ...

```

But then CYTHON_TRACE is sometimes not defined, which then lead to compile errors from the C/C++ compiler down the line. I could try to do

```

#ifndef CYTHON_TRACE

#define CYTHON_TRACE 0

#endif

```

in a cdef extern block, but that leads back to the original problem. CYTHON_TRACE would never be defined because this happens above the handling for it...

[da-woods]

I think you can use the "#cython: preliminary_late_includes = True" directive which pushes the extern blocks further down in the source file.

I think the plan is to eventually replace that macro with something better so do be prepared for it to break at some point. But that should probably work for now.

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