libsrp6a-sha512: fix environ for Apple

3rd_party/libsrp6a-sha512/t_misc.c

diff --git a/3rd_party/libsrp6a-sha512/t_misc.c b/3rd_party/libsrp6a-sha512/t_misc.c
index abd8e55..34b9509 100644
--- a/3rd_party/libsrp6a-sha512/t_misc.c
+++ b/3rd_party/libsrp6a-sha512/t_misc.c
@@ -77,7 +77,12 @@
  * tricks with variable ordering and sometimes define quirky
  * environment variables like $WINDOWID or $_.
  */
+#ifdef __APPLE__
+#include <crt_externs.h>
+#define environ (*_NSGetEnviron())
+#else
 extern char ** environ;
+#endif
 
 static void
 t_envhash(unsigned char * out)