'va_copy' not declared with --std=c++03

[neikeq]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
Ubuntu 17.04 64bit
g++ (Ubuntu 6.3.0-12ubuntu2) 6.3.0 20170406

Issue description:

Build error:

core/io/logger.cpp: In member function 'virtual void RotatedFileLogger::logv(const char*, __va_list_tag*, bool)':
core/io/logger.cpp:181:28: error: 'va_copy' was not declared in this scope
   va_copy(list_copy, p_list);
                            ^
core/io/logger.cpp: In member function 'virtual void CompositeLogger::logv(const char*, __va_list_tag*, bool)':
core/io/logger.cpp:235:28: error: 'va_copy' was not declared in this scope
   va_copy(list_copy, p_list);
                            ^

Steps to reproduce:
Run scons with CCFLAGS=--std=c++03

[neikeq]

cc @endragor