configure: Fix building with SASL on Windows

The Simple Authentication and Security Layer (SASL) library
re-defines the struct iovec on Win32 [*]. QEMU also re-defines
it in "qemu/osdep.h". The two definitions then clash on a MinGW
build.
We can avoid the SASL definition by defining STRUCT_IOVEC_DEFINED.
Since QEMU already defines 'struct iovec' if it is missing, add
the definition to vnc_sasl_cflags to avoid SASL re-defining it.

[*] https://github.com/cyrusimap/cyrus-sasl/blob/cyrus-sasl-2.1.27/include/sasl.h#L187



Cc: Alexey Pavlov <alexpux@gmail.com>
Cc: Biswapriyo Nath <nathbappai@gmail.com>
Reported-by: Youry Metlitsky <winaes@yandex.ru>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200309122454.22551-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>