Windows: don't set "-fstack-clash-protection" option

CYGWIN-GCC and MinGW-w64 crash with an "internal compiler error" when building libssh. This happens because -fstack-clash-protection option should not be set when using those platforms. Actually, Windows targets handle all this stuff natively, by calls to __chkstk_ms(), so this is simply redundant and it can be omitted. Since this option crashes GNU C, there is a bug open here:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90458

However, it would be worth to exclude WINDOWS and CYGWIN, to make already working all existing compilers. Afterall, it seems to be just useless on them.