Suppress unused parameter 'sig' warning from gcc

gcc now warns that sig is an unused parameter:

attach.c: In function 'win_change':
attach.c:99:16: warning: unused parameter 'sig' [-Wunused-parameter]
   99 | win_change(int sig)
      |            ~~~~^~~

1 files changed, 12 insertions, 0 deletions

diff --git a/dtach.h b/dtach.h
index c4d39cb..40d0879 100644
--- a/dtach.h
+++ b/dtach.h
@@ -18,6 +18,18 @@
 #ifndef dtach_h
 #define dtach_h
 
+#if defined(__has_attribute)
+#if __has_attribute(unused)
+#define ATTRIBUTE_UNUSED __attribute__((__unused__))
+#else
+#define ATTRIBUTE_UNUSED
+#endif
+#elif defined(__GNUC__) || defined(__clang__)
+#define ATTRIBUTE_UNUSED __attribute__((__unused__))
+#else
+#define ATTRIBUTE_UNUSED
+#endif
+
 #include <config.h>
 
 #include <errno.h>