Please build against OpenSSL 1.1 from EPEL7 for TLSv1.3 support

[prupert]

TLSv1.3 support is possible when building HAProxy against OpenSSL 1.1 from EPEL7, which has seen active support for a year now.

Nginx from EPEL7 has done the same.

[jeffsheltren]

Added in #10