This fixes segfault:
http://bugs.alpinelinux.org/issues/3750

diff --git a/sipsak.h b/sipsak.h
index 8f5d283..f9b1b49 100644
--- a/sipsak.h
+++ b/sipsak.h
@@ -57,7 +57,7 @@
 #endif
 
 #ifdef HAVE_STRCASESTR
-# define __USE_GNU
+# define _GNU_SOURCE
 # define STRCASESTR(s1,s2) strcasestr(s1,s2)
 #else
 # define STRCASESTR(s1,s2) strstr(s1,s2)