fix: updates for CentOS 7 support and feature #45

[AdrianDC]

Merge request for reference with the CentOS 7 compatibility fixes and rebased upon the #45 feature.

[AdrianDC]

@darrachequesne

Builds properly with this, used in production for a year :

docker run --rm -i -v "${PWD}:${PWD}" -w "${PWD}" centos:7.6.1810 <<EOF
yum install -y cmake gcc gcc-c++ make openssl-devel
cmake  ./
make install
EOF

[darrachequesne]

@AdrianDC thanks a lot for providing a way to reproduce the issue on CentOS 👍