winpthreads: Disable warnings for POSIX function names

Microsoft considers these POSIX function names to be deprecated, and
provides alternative names starting with an underscore. A deprecation
warning is raised if the old names are used, which can be disabled
with the _CRT_NONSTDC_NO_WARNINGS macro.

https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-3-c4996?view=msvc-170#posix-function-names

Signed-off-by: Antonin Décimo <antonin@tarides.com>
Signed-off-by: Martin Storsjö <martin@martin.st>