Change the openssl-devel package to libssl-devel. Cygwin still has an
"obsolete" (essentially virtual) package for openssl-devel, but it
doesn't seem to work properly on Appveyor's Cygwin install.
Protobuf >= 3.6.0 requires C++11, which I added support for last year.
But when I did that, I requested strict ANSI C++ compatibility, which
causes Cygwin/newlib's libc feature test macros to be set to disable
(at least) POSIX.1 extensions. Let the Autoconf macro use its default
instead (prefer GNU/extended C++, accept anything).
John Hood