chore: Makevars cleanup

[krlmlr]

This adds a dummy C++ file to ensure that a C++ linker is always selected. Also, we don't need to ship src/Makevars .

Rationale, from r-exts, https://cran.r-project.org/doc/manuals/r-release/R-exts.html#External-C_002b_002b-code:

Even fewer external libraries use C++ internally but present a C interface, such as rgeos. These require the C++ runtime library to be linked into the package’s shared object/DLL, and this is best done by including a dummy C++ file in the package sources.

This is essentially what we sent to CRAN, plus the dummy C++ file.

[codecov]

Codecov Report

Merging #671 (51d7ef0) into main (438c21d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #671   +/-   ##
=======================================
  Coverage   60.48%   60.48%           
=======================================
  Files          73       73           
  Lines       21608    21608           
=======================================
  Hits        13070    13070           
  Misses       8538     8538           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[szhorvat]

Can you add a link to https://cran.r-project.org/doc/manuals/r-release/R-exts.html#External-C_002b_002b-code in a comment?

[szhorvat]

Since this is following advice directly from the R manual, I guess it's fine. Please just keep that link to the manual in a comment in the sources.

This file is not going to be compiled, so there won't be issues with empty compilation units (which CRAN would complain about). No need for hacks like this.