When building PPP 2.5.2 against OpenSSL 3.0.x, the build fails in pppd/eap-tls.c because it uses deprecated ENGINE functions. OpenSSL 3.0 marks the ENGINE APIs as deprecated, and some calls appear to cause build failures depending on the toolchain.

eap-tls.c: In function 'eaptls_init_ssl':
eap-tls.c:411:9: warning: 'ENGINE_ctrl_cmd' is deprecated: Since OpenSSL 3.0
if (!ENGINE_ctrl_cmd( cert_engine, "LOAD_CERT_CTRL", 0, &cert_info, NULL, 0 ) )
^
eap-tls.c:536:13: warning: 'ENGINE_load_private_key' is deprecated: Since OpenSSL 3.0
pkey = ENGINE_load_private_key(pkey_engine, privkeyfile, transfer_pin, &cb_data);
^
eap-tls.c:542:13: warning: 'ENGINE_load_private_key' is deprecated: Since OpenSSL 3.0
pkey = ENGINE_load_private_key(pkey_engine, privkeyfile, NULL, NULL);
^
make[2]: *** [Makefile:819: all] Error 2
make[1]: *** [Makefile:505: all-recursive] Error 1