Test R package compilation with >= C++17

CRAN is now completely C++11 compliant, so demands removal of all CXX_STD statements for C++11. This then leaves the standard dependent on the local system. This repo demonstrates one issue when local systems are >= C++17, which fail to compile with use of std::set. The typical compile error looks something like this:

test.cpp:7:21:   required from here
/usr/include/c++/11/bits/stl_tree.h:770:15: error: static assertion failed: comparison object must be invocable as const
  770 |               is_invocable_v<const _Compare&, const _Key&, const _Key&>,
        |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/11/bits/stl_tree.h:770:15: note: ‘std::is_invocable_v<const by_a&, const TestStruct&, const TestStruct&>’ evaluates to false
make: *** [/opt/R/4.3.0/lib/R/etc/Makeconf:200: test.o] Error 1
ERROR: compilation failed for package ‘settest’

The solution requires explicitly specifying the set comparison operator as const, so changing from

bool operator () (const TestStruct& lhs, const TestStruct& rhs) { ... }

to

bool operator () (const TestStruct& lhs, const TestStruct& rhs) const { ... }