Force W32-specific POLL* constant values in meson
Force W32-specific POLL* constant values in meson

The winsock2-using test does work perfectly, however this is a new
thing that didn't exist in autotools-based builds of glib in the past.
Autotools builds used the generic case where values were just defined
to some agreed-upon numbers, and this is what all autotools-glib
binaries and binaries built against autotools-glib (since these
values go into public glibconfig.h header) use. At least one value,
G_POLLIN, is different, thus breaking ABI if some binaries are
built with autotools and the others are built with meson.

Force meson buildscript to use the same G_POLL* constant values
for Windows builds that autotools builds use.

https://bugzilla.gnome.org/show_bug.cgi?id=794687

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib