OSError('pkg-config is not installed',)

I'm getting this error, even though `pkg-config` is installed by Conda i.e. `Conda.add("pkg-config")` does nothing. This is for `FEniCS.jl`.

Error persists even if I downgrade to Julia 0.6.2, Conda 0.8.0, and PyCall 1.15.0 via `Pkg.pin`. What can I do to get a working system again?

```
ERROR: PyError (PyImport_ImportModule)
OSError('pkg-config is not installed',)
File "/Users/aterenin/.julia/packages/Conda/m7vem/deps/usr/lib/python3.6/site-packages/fenics/__init__.py", line 7, in
from dolfin import *
File "/Users/aterenin/.julia/packages/Conda/m7vem/deps/usr/lib/python3.6/site-packages/dolfin/__init__.py", line 140, in
from .fem.assembling import (assemble, assemble_system, assemble_multimesh,
File "/Users/aterenin/.julia/packages/Conda/m7vem/deps/usr/lib/python3.6/site-packages/dolfin/fem/assembling.py", line 34, in
from dolfin.fem.form import Form
File "/Users/aterenin/.julia/packages/Conda/m7vem/deps/usr/lib/python3.6/site-packages/dolfin/fem/form.py", line 12, in
from dolfin.jit.jit import dolfin_pc, ffc_jit
File "/Users/aterenin/.julia/packages/Conda/m7vem/deps/usr/lib/python3.6/site-packages/dolfin/jit/jit.py", line 15, in
if pkgconfig.exists("dolfin"):
File "/Users/aterenin/.julia/packages/Conda/m7vem/deps/usr/lib/python3.6/site-packages/pkgconfig/pkgconfig.py", line 62, in _wrapper
raise EnvironmentError("pkg-config is not installed")
```

Comienza reproduciendo la importación de FEniCS mediante PyCall y sigue el traceback a través de fenics/__init__.py, dolfin/__init__.py y dolfin/jit/jit.py. Comprueba cómo se relaciona Conda.add("pkg-config") con la comprobación de pkgconfig en Python y, después, documenta o verifica una configuración funcional reproducible.

Escrito por el modelo de indexación a partir del texto del issue.