Fix compilation on macOS Big Sur

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))

configure.ac

@@ -710,6 +710,14 @@ else
 	 OCFLAGS="${OCFLAGS} -Werror"])
 fi
 
+case $target in
+    *-*-darwin*)
+	AX_CHECK_COMPILE_FLAG([-Wno-expansion-to-defined],
+	[CFLAGS="${CFLAGS} -Wno-expansion-to-defined"
+	 OCFLAGS="${OCFLAGS} -Wno-expansion-to-defined"])
+	;;
+esac
+
 AX_CHECK_COMPILE_FLAG([-Werror=unused-result],
     [CFLAGS="${CFLAGS} -Wno-error=unused-result"
      OCFLAGS="${OCFLAGS} -Wno-error=unused-result"],