curl: Check for OpenSSL headers independently

Use check_include_file instead of check_include_file_concat to look
for OpenSSL headers.  They do not need to participate in a sequence
of dependent system headers.  Also they may cause winsock.h to be
included before ws2tcpip.h, causing the latter to not be detected
in the sequence.
Merge topic 'update-curl' into next

46bbf947 curl: Check for OpenSSL headers independently