[Cython] Automatically add include_dirs for externs (#1654)

[Jeroen Demeyer]

Dear Cython developers,

as part of a big effort to make SageMath less monolithic and make it 
part of a larger ecosystem, we are splitting off certain modules from 
SageMath as separate packages. cysignals was an early example of this.

Given that we use Cython a lot, it is really important for us that 
inter-package C and Cython interfaces work well. One blocking issue is 
https://github.com/cython/cython/pull/1654

That PR improves the way how "cdef extern from" files are found: it will 
automatically add a disutils include_dirs argument to the Extension if 
it's needed to find to extern file. It is needed whenever you have a 
package which installs .pxd and .h files in site-packages.


Cheers,
Jeroen.