[preprocessor] Dropping support for compilers that do not implement variadic macros

From: Edward Diener (eldiener_at_[hidden])
Date: 2020-09-14 22:52:33

In lieu with Boost's decision regarding dropping support for C++03 
compilation, I would like to drop support in the preprocessor library 
for preprocessors which do not implement variadic macros. Variadic 
macros were added in the C++11 standard, although a number of compilers 
supported them even before the official C++ standard. I will of course 
keep the same macro APIs, although deprecating non-variadic macro forms 
when alternate variadic macro forms exist. This will simplify Boost PP. 
Ideally, once Boost PP supports only preprocessors which implement 
variadic macros, Boost VMD can be added to Boost PP, for anyone who 
wants to use its functionality, and Boost VMD can then be retired.