With OpenSSL < 1.1 there is a potential for a race condition to occur.
This can cause the S3 engine to crash. The workaround is to add locking
callbacks to OpenSSL so that this doesn't happen.

https://curl.haxx.se/libcurl/c/threadsafe.html

There is a fix in libMariaS3 for this which when a certain flag is set
(HAVE_CURL_OPENSSL_UNSAFE) will add the required locks.

This patch adds CMake support so that the flag is set if it is found
that Curl is compiled with an unsafe OpenSSL version. For example Ubuntu
16.04 with libcurl4-openssl-dev.