Fix C++11 compilation errors.

Don't rely on friend method injection, and avoid prefixing constants with strings.

Change-Id: Ide6ab73d13ba3e3534e47323b21f6e5e3ea9d0ab
Reviewed-on: https://swiftshader-review.googlesource.com/1462
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>