I have successfully installed ctcdecode and the steps are as follows:
git clone --recursive https://github.com/parlance/ctcdecode.git
cd ctcdecode/third_party
tar -zxvf boost_1_67_0.tar.gz && tar -zxvf openfst-1.6.7.tar.gz
cd ../
python setup.py build
python setup.py install
pip install .