Automatically add include_dirs for externs

[jdemeyer]

Consider the following situation: some package Foo has installed files foo.pxd and header.h in .../python/site-packages/foo. The foo.pxd file contains cdef extern from "header.h" where header.h is installed in .../python/site-packages/foo/header.h

When another package Bar wants to cimport foo.foo, the cdef extern from "header.h" will become #include "header.h" in the C file. The problem is now that the C compiler will not be able to find this file.

The solution is obvious: whenever we look for a cdef extern from file, we add its directory to the include_dirs of the extension.

[embray]

Makes perfect sense to me.

[jdemeyer]

We're assuming in SageMath (https://trac.sagemath.org/ticket/22728) that this will be merged.

[jdemeyer]

An alternative solution to the problem would be to keep the Extension but replace filenames in the Cython-generated .c files. That is, Cython could turn

cdef extern from "foo.h":
    ...

into

#include "/path/to/foo.h"

Personally, I'm not convinced that this is a better solution, because I think it would be the only place in Cython where the code generation depends on data coming from the Extension object. For that reason, it might be harder to implement also.

[robertwb]

Yes, this looks good.

[scoder]

I had to apply the change in e9194f7 in order to avoid writing absolute path names into the C file. Hope it still works for everyone.