No libbgs.so file after "Building with Anaconda Python"

**Describe the bug**
After following the instructions posted at https://github.com/andrewssobral/bgslibrary/wiki/Wrapper:-Python#building-with-anaconda-python, no `libgsl.so` file is created, although `make` terminates with error code `0`. Running `python Demo.py` results in an error:

```
Traceback (most recent call last):
File "Demo.py", line 3, in
import bgs
ImportError: No module named bgs
```

**To Reproduce**
```bash
# Build OpenCV 3 from source
wget https://github.com/opencv/opencv/archive/3.4.3.zip
unzip $whatever && cd $whatever
mkdir build && cd build
cmake -D CMAKE_BUILD_TYPE=Release \
-D CMAKE_INSTALL_PREFIX="$HOME/local" \
-D PYTHON3_EXECUTABLE=/home/markham/miniconda3/envs/gen/bin/python \
-D PYTHON_INCLUDE_DIR=/home/markham/miniconda3/envs/gen/include \
-D PYTHON_LIBRARY=/home/markham/miniconda3/envs/gen/lib/libpython3.6m.so \
-D PYTHON3_NUMPY_INCLUDE_DIRS=/home/markham/miniconda3/envs/gen/include/boost/python/numpy \
..
make -j8 && make install
# Build bgslibrary from source
git clone --recursive https://github.com/andrewssobral/bgslibrary.git && \
cd bgslibrary && mkdir -p build && cd build && \
cmake -D OpenCV_DIR=$HOME/local/share/OpenCV -D BGS_PYTHON_SUPPORT=ON .. && \
make -j8 || exit 1
export PYTHONPATH="${PYTHONPATH}:$(pwd)/" # (add 'libbgs.so' to your Python path)
cd .. && python Demo.py
```

**Expected behavior**
I expected a `libbgs.so` file to be created and `Demo.py` to run without incident.

### System information
**Desktop (please complete the following information):**
- **OS Platform and Distribution (e.g., Linux Ubuntu 16.04)**: Linux Ubuntu 14.04.5 LTS
- **Python version (for issues related to the python wrapper)**: Python 3.6.6
- OpenCV 2.4.8
- GCC 4.9.4