Fix https-client compilation on Windows.

ssize_t is not defined. But using ssize_t for s in this context makes no
sense, since fread returns size_t.
Guard against EVENT_NOWIN32 being set during testing.

On windows all tests will fail if EVENT_NOWIN32 is set, since then there
will be no backend available.

Question is if we should simply disable the environment variable check on
Windows, since there's only one backend available anyway?
Check for OSX when checking for clang.
Merge remote-tracking branch 'joakimsoderberg/win32_fixes'