Fixes:

In file included from vev.c:36:
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/pthread.h:328:6: error: macro expansion producing 'defined' has undefined behavior [-Werror,-Wexpansion-to-defined]
 #if !_PTHREAD_SWIFT_IMPORTER_NULLABILITY_COMPAT
      ^
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/pthread.h:197:2: note: expanded from macro '_PTHREAD_SWIFT_IMPORTER_NULLABILITY_COMPAT'
         defined(SWIFT_CLASS_EXTRA) && (!defined(SWIFT_SDK_OVERLAY_PTHREAD_EPOCH) || (SWIFT_SDK_OVERLAY_PTHREAD_EPOCH < 1))