[xml] wsdks + winsock behaviours

[LRN <lrn1986 gmail com>]

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1) MS PSDK 7 includes winsock.h from windows.h only when compiling for MAC
winsock.h defines _WINSOCKAPI_, it is used as #include guard

2) mingw.org w32api includes winsock.h from windows.h only if WINVER <
0x400 (it could be reflecting that older MS PSDK used to include
winsock.h, i wouldn't know)
_WINSOCKAPI_ is never ever mentioned in any system header
3) mingw-w64 headers include winsock.h from windows.h always, unless
WIN32_LEAN_AND_MEAN or __CYGWIN__ are defined (or __CYGWIN__ and
__USE_W32_SOCKETS are defined at the same time)
winsock.h defines _WINSOCKAPI_, it is used as #include guard

libxml2 defines _WINSOCKAPI_ in its configure script if it detects
mingw as the host platform

result: CHAOS
1) for MS PSDK 7 it does practically nothing (unless it's MAC, in
which case it produces "_WINSOCKAPI_ redefined" warning)

2) for mingw.org w32api it does nothing (mingw.org doesn't use
_WINSOCKAPI_ macro at all)

3) for mingw-w64 it produces a warning "please include <winsock2.h>
before <windows.h>"


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