SM60 compiles with C++14, and headers contain code that is incompatible with C++11. We should upgrade to C++14 before upgrading Spidermonkey.
In terms of compiler support, this drops GCC < 5, Clang < 3.4, and VS13, so, in fact, pretty much nothing.
Can't actually use premake's cppdialect yet because of https://github.com/premake/premake-core/issues/984 (we're still on Alpha 12).