Update SConstruct

Add " -D_WIN32_WINNT=0x0501 " to link with getaddrinfo and
Add optimizations when build with MinGW on 32-bit Windows OS.

<<Since the getaddrinfo() is available to Windows XP onwards,
it is sufficient to use that windows version code as the value
of the constant mentioned above.>>

Compiling with scons --m32
Merge pull request #99 from yoniyoni/patch-2

Update SConstruct