[Rd] Makevars CXX_STD variable ignored when no *.cpp files in src/

[Radosław Piliszek]

Hello!

I might have found a bug in the way that R handles Makevars file when
building a package.

Value of variable CXX_STD is ignored - i.e. R does not use the correct
compiler/flags - if there are no *.cpp files directly in the src/
directory (e.g. all *.cpp are in subdirectories, and OBJECTS variable
is set accordingly). Adding a bogus *.cpp file fixes this issue.
However, this is not very obvious (I would dare saying it is not
obvious at all) and I spent quite a time checking what went wrong
after I organized my src files better. :-)

Kind regards,
Radosław Piliszek