Fix for 'inline' macro redefinition error in VS2012 C++

Currently when compiling C++ code, VS2012 errors on the keyword 'inline' being redefined.

This can be easily suppressed by adding ```# define _ALLOW_KEYWORD_MACROS``` within mrbconf.h.

Unfortunately this will also suppress the error for everything else being compiled, and **could** cause issues to someone unintentionally redefining other keywords.