Win32: i386 now assumed when crossbuilding (ticket #2416).

Previously, NGX_MACHINE was not set when crossbuilding, resulting in
NGX_ALIGNMENT=16 being used in 32-bit builds (if not explicitly set to a
correct value).  This in turn might result in memory corruption in
ngx_palloc() (as there are no usable aligned allocator on Windows, and
normal malloc() is used instead, which provides 8 byte alignment on
32-bit platforms).

To fix this, now i386 machine is set when crossbuilding, so nginx won't
assume strict alignment requirements.

auto/configure

@@ -44,6 +44,7 @@ if test -z "$NGX_PLATFORM"; then
 else
     echo "building for $NGX_PLATFORM"
     NGX_SYSTEM=$NGX_PLATFORM
+    NGX_MACHINE=i386
 fi
 
 . auto/cc/conf