Numpy build requires writeable source directory

[Araneidae]

The numpy build needs to mark the source directory as writeable in order to perform the build. For every other package this has been avoided somehow, I would like to continue to treat the source directory as pristine.

[blondejamtart]

When building 1.17.5 it fails cythonizing files (the error I get is for 'bounded_integers.pxd' but I suspect there are more files it's trying to cythonize into src). Also seems to rely on writing a version file into the source directory.