Index: external/bdwgc/os_dep.c
--- external/bdwgc/os_dep.c.orig
+++ external/bdwgc/os_dep.c
@@ -525,10 +525,7 @@ GC_INNER char * GC_get_maps(void)
   STATIC JMP_BUF GC_jmp_buf_openbsd;
 
 # ifdef THREADS
-#   include <sys/syscall.h>
-    EXTERN_C_BEGIN
-    extern sigset_t __syscall(quad_t, ...);
-    EXTERN_C_END
+    sigset_t signalmask;
 # endif
 
   /* Don't use GC_find_limit() because siglongjmp() outside of the      */
@@ -577,7 +574,9 @@ GC_INNER char * GC_get_maps(void)
 
 #   ifdef THREADS
       /* Due to the siglongjump we need to manually unmask SIGPROF.     */
-      __syscall(SYS_sigprocmask, SIG_UNBLOCK, sigmask(SIGPROF));
+      sigemptyset(&signalmask);
+      sigaddset(&signalmask, SIGPROF);
+      sigprocmask(SIG_UNBLOCK, &signalmask, NULL);
 #   endif
 
     sigaction(SIGSEGV, &old_segv_act, 0);