Be careful that you’ll download proper package from cloudflare repository. In my case, I’m using Apache 2.4 on Directadmin installation. At first, I downloaded wrong version of mod_cloudflare.c and was constantly getting errors while trying to build module.
Install mod_cloudflare using apxs
apxs -a -i -c mod_cloudflare.c
Check if module was installed into Apache
[root@server ~]# httpd -M | grep cloud cloudflare_module (shared)