CentOS 7's openssl version is 1.0, but I want to build NSD with openssl 1.1. The EPEL repository for CentOS 7 provides a newer openssl, and the RPM is called openssl11. It can be installed next to the older version without conflict. I can install the development files by installing the RPM called "openssl-devel". This puts the files into:

/usr/include/openssl11
/usr/include/openssl11/openssl
/usr/include/openssl11/openssl/aes.h
...

However, I'm finding it difficult to tell NSD to build and link against this. The --with-ssl configure option seems limited. It wants to look for the entire openssl 1.1 installation (headers, libraries) in a specific directory, but this is not how things are laid out in CentOS (or other Linux systems, for that matter). NSD's configure script doesn't seem to use pkg-config, because if it did, it would be as simple as running pkg-config --cflags --libs openssl11 to get the correct flags to pass to the compiler and linker. Is there any way I can get NSD to compile against this newer openssl under CentOS 7?