signal: guard __cdecl definition with #ifdef

Under mingw64:
  ../signal.c:88:0: warning: "__cdecl" redefined
   #define __cdecl

  <built-in>: note: this is the location of the previous definition

https://ci.appveyor.com/project/azat/libevent/builds/23321613#L427

1 files changed, 2 insertions, 0 deletions

diff --git a/signal.c b/signal.c
index 3f462950..89f5fc17 100644
--- a/signal.c
+++ b/signal.c
@@ -85,8 +85,10 @@
 #ifndef _WIN32
 /* Windows wants us to call our signal handlers as __cdecl.  Nobody else
  * expects you to do anything crazy like this. */
+#ifndef __cdecl
 #define __cdecl
 #endif
+#endif
 
 static int evsig_add(struct event_base *, evutil_socket_t, short, short, void *);
 static int evsig_del(struct event_base *, evutil_socket_t, short, short, void *);