[CORE] Azure core is not building on g++ 5

[vhvb1989]

g++ 5 is used in CI legs from cognitive lib.

base64.cpp is currently failing due to an ignored returned value.

Tasks:

• Fix the build on g++ 5
• Add g++ 5 to the CI/CD matrix

[RickWinter]

@vhvb1989 As part of this work we need to add G++ 5 to the CI/CD matrix

[vhvb1989]

@vhvb1989 As part of this work we need to add G++ 5 to the CI/CD matrix

Done.