build: add -fcf-protection=full to hardening options

Enables code instrumentation of control-flow transfers. Available in
GCC 8 and Clang 7.

This option is now on by default in Ubuntu GCC as of 19.10.

configure.ac

@@ -785,6 +785,7 @@ if test x$use_hardening != xno; then
   AX_CHECK_COMPILE_FLAG([-Wstack-protector],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -Wstack-protector"])
   AX_CHECK_COMPILE_FLAG([-fstack-protector-all],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-protector-all"])
 
+  AX_CHECK_COMPILE_FLAG([-fcf-protection=full],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fcf-protection=full"])
   dnl When enable_debug is yes, all optimizations are disabled.
   dnl However, FORTIFY_SOURCE requires that there is some level of optimization, otherwise it does nothing and just creates a compiler warning.
   dnl Since FORTIFY_SOURCE is a no-op without optimizations, do not enable it when enable_debug is yes.