Add a pre-C++11 constexpr compatibility macro.

Needed for cases where something should be constexpr if possible, but
not being constexpr is fine if in pre-C++11 code (such as a const
static float member variable).

Bug: 18466763
Change-Id: I635d062575ba2fbc4cbe3a89f730128c404d95e1
Merge "Add a pre-C++11 constexpr compatibility macro."