configure: error out if OpenSSL wasn't detected when asked for

If --with-ssl is used and configure still couldn't enable SSL this
creates an error instead of just silently ignoring the fact.

Suggested-by: Isaiah Norton
Fixes #3824

configure.ac

@@ -1889,6 +1889,11 @@ if test -z "$ssl_backends" -o "x$OPT_SSL" != xno &&
   test -z "$ssl_msg" || ssl_backends="${ssl_backends:+$ssl_backends, }$ssl_msg"
 fi
 
+if test X"$with_ssl" != Xno &&
+  test "$OPENSSL_ENABLED" != "1"; then
+  AC_MSG_ERROR([--with-ssl was given but OpenSSL could not be detected])
+fi
+
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