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>

configure

@@ -3367,7 +3367,9 @@ if test "$vnc" = "yes" && test "$vnc_sasl" != "no" ; then
 int main(void) { sasl_server_init(NULL, "qemu"); return 0; }
 EOF
   # Assuming Cyrus-SASL installed in /usr prefix
-  vnc_sasl_cflags=""
+  # QEMU defines struct iovec in "qemu/osdep.h",
+  # we don't want libsasl to redefine it in <sasl/sasl.h>.
+  vnc_sasl_cflags="-DSTRUCT_IOVEC_DEFINED"
   vnc_sasl_libs="-lsasl2"
   if compile_prog "$vnc_sasl_cflags" "$vnc_sasl_libs" ; then
     vnc_sasl=yes