Add R 3.6.0

[glin]

This was pretty straightforward, except for CentOS 6 (!!). Packages with C++11 code couldn't be installed:

+ R CMD INSTALL /test/testpkg --clean
* installing to library '/opt/R/3.6.0/lib/R/library'
* installing *source* package 'testpkg' ...
** using staged installation
** libs
Error in .shlib_internal(args) :
  C++11 standard requested but CXX11 is not defined
* removing '/opt/R/3.6.0/lib/R/library/testpkg'
Makefile:31: recipe for target 'test-3.6-centos6' failed
make: *** [test-3.6-centos6] Error 1

In previous versions of R (3.4, 3.5), this CXX11 Make variable was set to CXX11 = g++ in the $R_HOME/etc/Makeconf file. Since R 3.6.0, I think this is no longer set if the C++ compiler used to compile R doesn't support C++11.

The options selected for the C++ compiler default to the C++11 standard if supported, otherwise to the C++98 standard.
https://cran.r-project.org/doc/manuals/r-devel/NEWS.html

For the use of C++11 and later in R packages see the ‘Writing R Extensions’ manual. Currently the default C++ standard is that of the compiler used: from R 3.6.0 it will be C++11 if supported by the compiler.
https://cran.r-project.org/doc/manuals/R-admin.html#C_002b_002b-Support

---

We could compile R on CentOS 6 with the newer C++ compiler instead, but I think it'd be better to just add the proper configuration variables here. I've added a CentOS 6 specific Dockerfile template that sets these in the system-wide $R_HOME/etc/Makevars.site. The values were copied from the defaults on CentOS 7, and similar to what others have recommended. The tests pass now, but I think we should try building CRAN on 3.6-centos6 to truly test it later.

# CXX11 Make variables

# 3.6-centos6
$ docker run --rm rstudio/r-base:3.6-centos6 cat $(R RHOME)/etc/Makeconf | grep CXX11
CXX11 = 
CXX11FLAGS =  $(LTO)
CXX11PICFLAGS = 
CXX11STD = 
SHLIB_CXX11LD = $(CXX11) $(CXX11STD)
SHLIB_CXX11LDFLAGS = -shared

# 3.6-centos7
$ docker run --rm rstudio/r-base:3.6-centos7 cat $(R RHOME)/etc/Makeconf | grep CXX11
CXX11 = g++
CXX11FLAGS = -g -O2 $(LTO)
CXX11PICFLAGS = -fpic
CXX11STD = -std=gnu++11
SHLIB_CXX11LD = $(CXX11) $(CXX11STD)
SHLIB_CXX11LDFLAGS = -shared

References

• Similar issue on MRO 3.4 with the same solution: Package installation error: "C++11 standard requested but CXX11 is not defined" microsoft/microsoft-r-open#26 (comment)
• Docs for each Make variable: https://github.com/wch/r-source/blob/R-3-6-branch/config.site#L255
• C++ support in R: https://cran.r-project.org/doc/manuals/R-admin.html#C_002b_002b-Support