Fix compilation errors with libstdc++v3 for AVR target and allow --enable-libstdcxx

[Felipe Magno de Almeida]

Hello,

I've fixed a few compilation errors with libstdc++v3 with AVR-Libc and
was even able to use boost.spirit x3 with small patches to boost as
well.

Attached is the patch.

Please don't hesitate to ask modifications for upstream inclusion.

Right now std::cout is not working (and just include'ing <iostream>
makes the final executable 100k bigger), however being able to use all
C++11 and C++14 goodies from the STL is already really great.

Kind regards,
-- 
Felipe Magno de Almeida
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