Merge pull request #1426 from YosysHQ/mmicko/fix_environ

Define environ, fixes #1424

frontends/rpc/rpc_frontend.cc

diff --git a/frontends/rpc/rpc_frontend.cc b/frontends/rpc/rpc_frontend.cc
index b4b2fa3..83e1353 100644
--- a/frontends/rpc/rpc_frontend.cc
+++ b/frontends/rpc/rpc_frontend.cc
@@ -34,6 +34,8 @@
 #include "libs/sha1/sha1.h"
 #include "kernel/yosys.h"
 
+extern char **environ;
+
 YOSYS_NAMESPACE_BEGIN
 
 #if defined(_WIN32)