BUILD: ssl: improve SSL_CTX_set_ecdh_auto compatibility

BUILD: ssl: improve SSL_CTX_set_ecdh_auto compatibility

SSL_CTX_set_ecdh_auto() is not defined when OpenSSL 1.1.1 is compiled
with the no-deprecated option. Remove existing, incomplete guards and
add a compatibility macro in openssl-compat.h, just as OpenSSL does:

https://github.com/openssl/openssl/blob/bf4006a6f9be691ba6eef0e8629e63369a033ccf/include/openssl/ssl.h#L1486

This should be backported as far as 2.0 and probably even 1.9.

(cherry picked from commit a26d1e13245a760bd422e4e4b6a85cc17f9f0a60)
Signed-off-by: Willy Tarreau <w@1wt.eu>