Find out why recipes are not reproducible and if we can fix them

[tdejager]

From preliminary results, some recipes already not seem reproducible e.g pthreads-stubs

Either we need to:

• Fix them, with a patch or w/e so they are reproducible or,
• Mark them as not reproducible and show that on the README somehow.

[nichmor]

Closing because we create separate issue per recipe to track them better like this: https://github.com/prefix-dev/reproducible-builds/issues/75