Make CXX17 the default

[jeroen]

This matches config of R 4.3 on other platforms, for example on Linux:

# R CMD config CXX
g++ -std=gnu++17

# R CMD config CXXFLAGS
-g -O2

This fixes r-wasm/webr#347.

Ps: it is unclear to me how webr-vars.mk related to in this package related to the one in webr. Do we fix it there as well?

[georgestagg]

Thanks!

it is unclear to me how webr-vars.mk related to in this package related to the one in webr. Do we fix it there as well?

That version is used for building the extra packages provided in the webr/packages directory as part of the webR build process. In principle, we should probably fix it there too, but at the moment the only extra package included is the webr support package, which does not use any C++ code anyway.