If _WIN32_WINNT is unset, we force it to a new enough value to
make sure the necessary definitions are visible.

When targeting Windows Phone or Windows RT, _WIN32_WINNT should
be at least 0x0602 - otherwise the windows headers themselves
can cause errors (which technically are bugs in the headers).

Raising this value here shouldn't hurt; the alternative would
be to not touch it at all if WINAPI_FAMILY is set to phone/app,
or to force setting it to 0x0602 in configure if unset (for phone/app).
Signed-off-by: Martin Storsjö <martin@martin.st>