Add support for passing additional flags to spiped via new
spiped_pipe_X_flags variables. Replace USE_OPENSSL with
USES ssl. [1]

While I'm here, clarify the comment about needing C99 but
preferring a compiler with AESNI support; it applies to the
USES compiler:c11 part but not tar:tgz or ssl.

PR: 215501
Submitted by: Dave Cottlehuber