But wait. Compiling plugins on a brand new CentOS install drops an error:
check_http.c:312:9: error: ‘ssl_version’ undeclared (first use in this function)
After half a minute long interaction with my friend Google, I found that I should do a simple apt-get install libssl-dev to exorcise the the problem. Okay but I have no such package on CentOS and don't want to go with the nuisance of making one from source. Openssl-devel package doesn't help either.
Huhh. What a mess. Solution: have a look at the pic. Fix that file in plugins/ according to this and ./configure && make all again.