[Rd] Experimental CXX_STD problem in R 3.4

[Jeroen Ooms]

R 3.4 has 'experimental' support for setting CXX_STD to CXX98 / CXX11
/ CXX14 / CXX17.

However on most platforms, the R configuration seems to leave the
CXX1Y and CXX1Z fields blank in "${R_HOME}/etc/Makeconf" (rather than
falling back on default CXX). Therefore specifying e.g CXX_STD= CXX14
will fail build with cryptic errors (due to compiling with CXX="")

I don't think this is intended? Some examples from r-devel on Windows:

CXX11: https://win-builder.r-project.org/R8gg703OQSq5/
CXX98: https://win-builder.r-project.org/mpVfXxk79FaN/
CXX14: https://win-builder.r-project.org/L3BSMgAk4cQ7/
CXX17: https://win-builder.r-project.org/3ETZXrgkg77I/

Similar problems appear on Linux. I think the problem is that Makeconf
contains e.g:

CXX1Z =
CXX1ZFLAGS =
CXX1ZPICFLAGS =
CXX1ZSTD =

When CXX_STD contains any other unsupported value (e.g. CXX24) R
simply falls back on the default CXX configuration. The same should
probably happen for e.g. CXX17 when CXX1Z is unset in Makeconf?