Fix Error C2146 and error C4430 in MSVC.

[joe2018Outlookcom]

Replace ‘constexpr’ with 'const', for Visual Studio 2013.
Fixes #853
CLA: trivial

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[joe2018Outlookcom]

CLA: trivial

[joe2018Outlookcom]

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[pleroy]

The documentation says that the library requires C++11 to build. Why are we reverting to C++03, and why do we believe that this is the only problem?

The main support end date for VS 2013 was April 9, 2019. Let it rest in peace.

[LebedevRI]

Relevant: eb8cbec
I'm going to close this.