Failed to install populus on Windows (missing 'pkg-config')

* Populus Version: 1.4.1
* OS: win

### What was wrong?

'pkg-config' is required to install this package.

> C:\utils\populus>pip install -r requirements.txt
> Collecting click>=5.1 (from -r requirements.txt (line 1))
> Using cached click-6.6.tar.gz
> Collecting configparser==3.5.0 (from -r requirements.txt (line 2))
> Using cached configparser-3.5.0.tar.gz
> Collecting contextlib2>=0.5.4 (from -r requirements.txt (line 3))
> Using cached contextlib2-0.5.4-py2.py3-none-any.whl
> Collecting eth-testrpc>=0.8.0 (from -r requirements.txt (line 4))
> Using cached eth_testrpc-0.8.7-py3-none-any.whl
> Collecting ethereum-tester-client>=1.1.0 (from -r requirements.txt (line 5))
> Using cached ethereum_tester_client-1.2.5-py3-none-any.whl
> Collecting gevent>=1.1.2 (from -r requirements.txt (line 6))
> Using cached gevent-1.1.2-cp35-cp35m-win_amd64.whl
> Collecting py-geth>=1.1.0 (from -r requirements.txt (line 7))
> Using cached py-geth-1.4.1.tar.gz
> Collecting py-solc>=0.6.0 (from -r requirements.txt (line 8))
> Using cached py_solc-0.6.0-py3-none-any.whl
> Collecting pylru>=1.0.9 (from -r requirements.txt (line 9))
> Using cached pylru-1.0.9.tar.gz
> Collecting pysha3>=0.3 (from -r requirements.txt (line 10))
> Using cached pysha3-0.3.zip
> Collecting pytest>=2.7.2 (from -r requirements.txt (line 11))
> Using cached pytest-3.0.4-py2.py3-none-any.whl
> Collecting toposort>=1.4 (from -r requirements.txt (line 12))
> Using cached toposort-1.5-py2.py3-none-any.whl
> Collecting web3>=2.7.0 (from -r requirements.txt (line 13))
> Using cached web3-3.2.0-py3-none-any.whl
> Collecting json-rpc>=1.10.3 (from eth-testrpc>=0.8.0->-r requirements.txt (line 4))
> Using cached json_rpc-1.10.3-py2.py3-none-any.whl
> Collecting ethereum>=1.5.2 (from eth-testrpc>=0.8.0->-r requirements.txt (line 4))
> Using cached ethereum-1.6.0.tar.gz
> Collecting rlp>=0.4.4 (from eth-testrpc>=0.8.0->-r requirements.txt (line 4))
> Using cached rlp-0.4.7.tar.gz
> Collecting Werkzeug>=0.11.10 (from eth-testrpc>=0.8.0->-r requirements.txt (line 4))
> Using cached Werkzeug-0.11.11-py2.py3-none-any.whl
> Collecting greenlet>=0.4.9 (from gevent>=1.1.2->-r requirements.txt (line 6))
> Using cached greenlet-0.4.10-cp35-cp35m-win_amd64.whl
> Collecting colorama; sys_platform == "win32" (from pytest>=2.7.2->-r requirements.txt (line 11))
> Using cached colorama-0.3.7-py2.py3-none-any.whl
> Collecting py>=1.4.29 (from pytest>=2.7.2->-r requirements.txt (line 11))
> Using cached py-1.4.31-py2.py3-none-any.whl
> Collecting geventhttpclient>=1.3.1 (from web3>=2.7.0->-r requirements.txt (line 13))
> Using cached geventhttpclient-1.3.1.tar.gz
> Collecting ethereum-abi-utils>=0.2.1 (from web3>=2.7.0->-r requirements.txt (line 13))
> Using cached ethereum-abi-utils-0.2.2.tar.gz
> Collecting PyYAML (from ethereum>=1.5.2->eth-testrpc>=0.8.0->-r requirements.txt (line 4))
> Using cached PyYAML-3.12-cp35-cp35m-win_amd64.whl
> Collecting scrypt (from ethereum>=1.5.2->eth-testrpc>=0.8.0->-r requirements.txt (line 4))
> Using cached scrypt-0.8.0-cp35-cp35m-win_amd64.whl
> Collecting pyethash (from ethereum>=1.5.2->eth-testrpc>=0.8.0->-r requirements.txt (line 4))
> Using cached pyethash-0.1.27.tar.gz
> Collecting pycryptodome>=3.3.1 (from ethereum>=1.5.2->eth-testrpc>=0.8.0->-r requirements.txt (line 4))
> Downloading pycryptodome-3.4.3-cp35-cp35m-win_amd64.whl (7.6MB)
> 100% |################################| 7.6MB 123kB/s
> Collecting repoze.lru (from ethereum>=1.5.2->eth-testrpc>=0.8.0->-r requirements.txt (line 4))
> Using cached repoze.lru-0.6.tar.gz
> Collecting secp256k1 (from ethereum>=1.5.2->eth-testrpc>=0.8.0->-r requirements.txt (line 4))
> Using cached secp256k1-0.13.2.tar.gz
> Complete output from command python setup.py egg_info:
> 'pkg-config' is required to install this package. Please see the README for details.
>
> ----------------------------------------
> Command "python setup.py egg_info" failed with error code 1 in C:\Users\Lenovo\AppData\Local\Temp\pip-build-dtt61rik\secp256k1\
>
>

#### Cute Animal Picture

http://www.flashgames247.com/images/16403.jpg

Start by reproducing the Windows installation from requirements.txt and inspect the secp256k1 setup output, especially its README reference. Determine whether the project can install this dependency on Windows or needs a documented prerequisite, then validate the chosen resolution with the same pip command.

Written by the indexing model from the issue text.