R CMD config CXXCPP has been deprecated: it is not used by R itself and
there are several things wrong with the standard autoconf detection code:
- If CXXCPP is set by the user, it is not tested. It could be empty,
which AFAICS none of you allow for.
- The code looks at $CXX -E and /lib/cpp in turn, and tests a system C
header without consulting CPPFLAGS. /lib/cpp is unlikely to find C++
headers, and we have seen instances where without CPPFLAGS it did not
find C headers.
- It is the setting for the default C++ compiler, in R-devel C++11 but
not specified in earlier R (even 3.6.x could be C++98).
It would be better to use $(CXX) -E (or $(CXX11) etc) or test for yourself.
BR wrote: