Add support for -std=c++17 & -std=c++2a flag

[praetorian20]

gcc/clang C++ standard version flags now support the -std=c++17 &
-std=c++2a flags, along with their "gnu" counterparts.

[coveralls]

Coverage decreased (-0.1%) to 92.638% when pulling 417add6 on praetorian20:develop into 87b4e12 on gccxml:develop.

[praetorian20]

The travis-ci failure is due to a setup issue, not the code change.

[iMichka]

The patch looks good. I am working on fixing the CI here: #98. Once this is done I'll have a closer look to your changes :)

[praetorian20]

Thanks. I added some new is_cxx* functions but I don't think any other than is_cxx11* are actually used. So if you want I can delete the rest.

[iMichka]

I fixed the CI, you can rebase and force push this.

[praetorian20]

I deleted all the unused is_cxx* functions. Let me know if that's OK, I can add them back in if you prefer.

[praetorian20]

@iMichka, sorry to bother you, but if the patch looks OK, would you please consider merging and creating a release? It would save me the trouble of having to patch pygccxml locally.

[praetorian20]

Hi @iMichka, any update on this?

[iMichka]

Hey, really sorry, got stuck with other projects. I really need to find more time to work on this next pygccxml release.

Could you add the is_cxx* functions back? They may not be used inside pygccxml, but they are not part of the API and somebody may have used them. If we want to remove them, they need to go through a deprecation cycle (with a deprecation warning in 2.0.0, and a removal in 2.1.0).

So please just add them back, thanks. Once this is done I will merge this right away.

[praetorian20]

Done, added the functions back. I haven't added new is_cxx17 and is_cxx2a` functions because it would be nice to get rid of all the unused ones at some point.

[iMichka]

Thanks @praetorian20. I will be on vacation and without internet connection during the 2 next weeks. I hope I can work a little bit on pygccxml soon so that I can get this 2.0.0 release out.