qemu-ga: Fix use of environ on Darwin

Use _NSGetEnviron() helper to access the environment.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Cc: Charlie Somerville <charlie@charliesomerville.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qga/commands-posix.c

@@ -22,8 +22,13 @@
 #include "host-utils.h"
 
 #ifndef CONFIG_HAS_ENVIRON
+#ifdef __APPLE__
+#include <crt_externs.h>
+#define environ (*_NSGetEnviron())
+#else
 extern char **environ;
 #endif
+#endif
 
 #if defined(__linux__)
 #include <mntent.h>