Use X509_check_host() in OpenSSL 1.1.0 and later and don't use it

for OpenSSL 1.0.2 anymore, 1.0.2 will use the fallback version.
This changes the include file.

(OpenSSL 1.0.2 is out of support since Jan 1 2020 so one may wonder
 why care at all, but i'm trying not to break that during minor
 UnrealIRCd releases)

autoconf/m4/unreal.m4

@@ -327,7 +327,7 @@ AC_MSG_CHECKING([for X509_check_host in SSL library])
 AC_LANG_PUSH(C)
 SAVE_LIBS="$LIBS"
 LIBS="$LIBS $CRYPTOLIB"
-AC_TRY_LINK([#include <openssl/ssl.h>],
+AC_TRY_LINK([#include <openssl/x509v3.h>],
 	[X509_check_host(NULL, NULL, 0, 0, NULL);],
 	has_function=1,
 	has_function=0)