implement compatibility shim for __warn_references

This will allow us to warn about deprecated function references at link-time.

include/Makefile.am

@@ -21,6 +21,7 @@ noinst_HEADERS += machine/endian.h
 noinst_HEADERS += netinet/in.h
 noinst_HEADERS += netinet/tcp.h
 
+noinst_HEADERS += sys/cdefs.h
 noinst_HEADERS += sys/ioctl.h
 noinst_HEADERS += sys/mman.h
 noinst_HEADERS += sys/select.h

include/sys/cdefs.h

@@ -0,0 +1,31 @@
+/*
+ * Public domain
+ * sys/cdefs.h compatibility shim
+ */
+
+#ifndef LIBCRYPTOCOMPAT_SYS_CDEFS_H
+#define LIBCRYPTOCOMPAT_SYS_CDEFS_H
+
+#ifdef _WIN32
+
+#define __warn_references(sym,msg)
+
+#else
+
+#include_next <sys/cdefs.h>
+
+#ifndef __warn_references
+
+#if defined(__GNUC__)  && defined (HAS_GNU_WARNING_LONG)
+#define __warn_references(sym,msg)          \
+  __asm__(".section .gnu.warning." __STRING(sym)  \
+         " ; .ascii \"" msg "\" ; .text");
+#else
+#define __warn_references(sym,msg)
+#endif
+
+#endif /* __warn_references */
+
+#endif /* _WIN32 */
+
+#endif /* LIBCRYPTOCOMPAT_SYS_CDEFS_H */