MSVC: fix poll-related macro redefines
This seems to be related to the poll-emulation... I see that these things
are guarded by an "#if(_WIN32_WINNT >= 0x0600)" in <winsock2.h>, which
means it's supported for Windows Vista and above... We still support
Windows XP, so it seems someone has set this too high :)
I'd prefer to set this from the Makefile, but this generates a warning in
compat/win32/poll.c about redefining a macro (poll.c wants it to be 0x502,
which is Windows XP with SP2, rather than 0x501 which is normal Windows
XP).
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>