This code is C++ only. There isn't a single C++ compiler that fails to
understand the "inline" keyword, since it's required by C++98. Any
compiler older than C++98 is likely to choke on the template usage
further down, so this isn't necessary.

Moreover, the C++ standard says you cannot define macros.
[lib.macro.names] says "Nor shall such a translation unit define macros
for names lexically identical to keywords." -- technically, it's a
promise that the Standard Library headers won't do it, the wording means
that the entire translation unit won't do it, which implies no source
can do it.

MSVC complains about it:
fatal error C1189: #error : The C++ Standard Library forbids macroizing
keywords. Enable warning C4005 to find the forbidden macro.

Author: Thiago Macieira <thiago.macieira@intel.com>