implement compatibility shim for __warn_references

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

configure.ac

@@ -88,6 +88,18 @@ case $host_cpu in
 		;;
 esac
 
+AC_MSG_CHECKING([if .gnu.warning accepts long strings])
+AC_LINK_IFELSE([AC_LANG_SOURCE([[
+extern void SSLv3_method();
+__asm__(".section .gnu.warning.SSLv3_method; .ascii \"SSLv3_method is insecure\" ; .text");
+int main() {return 0;}
+]])], [
+    AC_DEFINE(HAS_GNU_WARNING_LONG, 1, [Define if .gnu.warning accepts long strings.])
+    AC_MSG_RESULT(yes)
+], [
+   AC_MSG_RESULT(no)
+])
+
 AC_ARG_ENABLE([asm],
 	AS_HELP_STRING([--disable-asm], [Disable assembly]))
 AM_CONDITIONAL([OPENSSL_NO_ASM], [test "x$enable_asm" = "xno"])

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 */