Remove PKG_CXXFLAGS = CXX11STD from Makevars

[mpadge]

CRAN will now insist on that being removed anyway, as confirmed yesterday in email from UL which explained that their check is now just grep -r CXX11, so this package will fail in current form.

[kevinushey]

Okay, I'll wait until we receive that communication from CRAN then?

[eddelbuettel]

FWIW I sort-of did it "rolling" in all my repos. It is a bit of a mixed bag: I have a few where I have set C++14 (because of Boost libraries or alike) which leads to a nag, as does setting C++17. But they clearly no longer want us to set C++11 unless required so just removing worked for me. So my $0.02 would be to remove prior to next upload.

[mpadge]

But carefully. I discovered this trying to debug another pkg. With no STD, most systems seem to currently default to 17, and that has some breaking changes in precompiled bits/std headers.