`b2` build fails on gcc 4.8

[Lastique]

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Environment and version details

• Operating System+version: Ubuntu 14.04.6 (Drone image)
• Compiler+version: gcc 4.8.4
• Shell: bash
• B2 Version: Boost develop (commit 9890997)

Brief problem description

Boost.Config Drone CI fails to compile b2 with the following error:

value.cpp: In static member function 'static b2::value* b2::value::make(b2::object*)':
value.cpp:263:59: error: call of overloaded 'save(b2::object*&)' is ambiguous
  return value_cache().save<value_object, value_object>(obj);
                                                           ^
value.cpp:263:59: note: candidates are:
value.cpp:206:12: note: b2::value* b2::safe_value_cache::save(Args ...) [with Test = b2::value_object; Val = b2::value_object; Args = {b2::object*}; b2::value_ptr = b2::value*]
  value_ptr save(Args... args)
            ^
value.cpp:218:12: note: b2::value* b2::safe_value_cache::save(b2::object*) [with Test = b2::value_object; Val = b2::value_object; b2::value_ptr = b2::value*]
  value_ptr save(object * obj)
            ^

Steps to reproduce the issue

Run Boost.Config Drone CI.

[grafikrobot]

This was fixed in 82b627a