Comment out the -DUSE_OPENSSL flag from CFLAGS in Makefile and recompile.
This will cripple zxid from security perspective because it will no longer be able to verify or generate digital signatures. Unless your environment does not need trust and security, or you understand thoroughly how to provide trust and security by other means, it is a very bad idea to compile without OpenSSL.
N.B. Compiling, or not, zxid with OpenSSL does not affect whether your web server will use SSL or TLS. Unless you know what you are doing, you should be using SSL at web server layer. Given that SSL is used at web server layer, the memory footprint savings you would gain from compiling zxid without OpenSSL may be negligible if you use dynamic linking.