Build fix: missing type traits on gcc < 5.0

gcc versions before 5.0 lack some type traits defined in C++11. Since in
this instance we use the trait to prevent misuses of certain functions
to statically assert at build time and not to generate different code
based on this property, it is preferable to simply set value to true and
allow the code to compile. Anyone using a contemporary compiler will
catch the error.

I have not replaced the type trait checks with macros using gcc-specific
checks (__has_trivial_copy(), for example) since their semantics diverge
subtly from the standard and their use could introduce bugs.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id57cc1cff67847c725f75eb3404443732de1c531