Update doctest to work with ARM DS5-compiler

[tomasnilefrost]

While trying to use doctest with the ARM DS5-compiler I ran into some issues with the standard library methods (free, malloc, sprintf, printf etc) not being present.

ARM DS5 puts all of the standard library functions in the std-namespace and do not put anything in the global namespace.

See for instance the blog-post: https://developers.redhat.com/blog/2016/02/29/why-cstdlib-is-more-complicated-than-you-might-think/
"You must not assume that adds any names to the global namespace, and you must not assume that <xxx.h> adds any names to namespace std."

The solution is to explicitly point out for instance std::printf and not rely on the standard library for the specific compiler to export these symbols in the global namespace.

This might actually also be related to a previous pull-request ( #37 ) but I haven't investigated it further.

[onqtam]

Thanks for the PR (made against the dev branch!!!)

The previous issue really seems related to this. I finally understand the difference between the headers - thanks for the redhat article.

I'm merging this as it is because it's the best option - even though a solution could be to just do using std::strcpy; and etc - or just include the headers in a C style - #include <xxx.h>. I'll also move away from most of these C functions as I move to streams and etc.

I will give a hard deadline for version 1.2 of 1 month from now (I keep postponing it...) - so only then will these changes appear in the master branch.

[tomasnilefrost]

Thanks for accepting the pull request. Great job with doctest!

Regarding solution of using std::strcpy, it is probably best avoided since that would mean that all code including doctest.h would also get a polluted global namespace of these using-declarations.