BLD Add DISTDIR variable. ### Description

Adds `DISTDIR` env variable that points to the directory where libraries should be copied to when building shared libraries with recipe.

### Checklists

- [x] Add new / update outdated documentation

# setup: import numpy as np; r = np.arange(1000000, dtype=float)
# run: rosen(r)
import numpy as np

# pythran export rosen(float[])


def rosen(x):
    t0 = 100 * (x[1:] - x[:-1] ** 2) ** 2
    t1 = (1 - x[:-1]) ** 2
    return np.sum(t0 + t1)

# setup: import numpy as np; r = np.arange(1000000, dtype=float)
# run: rosen(r)
import numpy as np

# pythran export rosen(float[])


def rosen(x):
    t0 = 100 * (x[1:] - x[:-1] ** 2) ** 2
    t1 = (1 - x[:-1]) ** 2
    return np.sum(t0 + t1)
