linux-user: Fix incorrect use of feature-test-macros

The _POSIX_C_SOURCE and _XOPEN_SOURCE macros are used by the
application to indicate to libc which declarations it should expose.
Since qemu does not define them anywhere, it does not make sense
to check their value.

Instead, since the intent is to determine whether the host struct
stat supports the st_*tim fields, use the configure test result
which does exactly that.

Signed-off-by: Michael Forney <mforney@mforney.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210526035531.7871-1-mforney@mforney.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>