Rstanarm does not build on windows because of CXX14 flag

Summary:

Due to the requirement of CXX14 since bd717c0 the package does not compile, giving the following error:

Error in .shlib_internal(args) : 
  C++14 standard requested but CXX14 is not defined

Description:

Latest version of RTools is installed (3.5.0.4), seems to contain GCC version 4.9.3, which according to this link does not support all features of C++14.

NB: I found this issue because I wanted to create a package with precompiled models via rstantools, which seems to depend on the makevars.win from rstanarm. Here, changing CXX14 to CXX11 seems to fix the issue.

Reproducible Steps:

devtools::install_github("stan-dev/rstanarm")

Downloading GitHub repo stan-dev/rstanarm@master
√  checking for file 'C:\Users\5507553\AppData\Local\Temp\Rtmp0YKDkX\remotes23382edd7cb8\stan-dev-rstanarm-a37f9b3/DESCRIPTION' ...
-  preparing 'rstanarm': (683ms)
√  checking DESCRIPTION meta-information ... 
-  cleaning src
-  running 'cleanup.win' (789ms)
-  checking for LF line-endings in source and make files and shell scripts (847ms)
-  checking for empty or unneeded directories
-  looking to see if a 'data/datalist' file should be added
-  building 'rstanarm_2.18.2.tar.gz'
   Warning: file 'rstanarm/cleanup' did not have execute permissions: corrected
   
* installing *source* package 'rstanarm' ...
** libs

*** arch - i386
Error in .shlib_internal(args) : 
  C++14 standard requested but CXX14 is not defined
* removing 'C:/Program Files/R/R-3.5.1/library/rstanarm'
* restoring previous 'C:/Program Files/R/R-3.5.1/library/rstanarm'
In R CMD INSTALL
Error in i.p(...) : 
  (converted from warning) installation of package ‘C:/Users/5507553/AppData/Local/Temp/Rtmp0YKDkX/file23384a3c5fec/rstanarm_2.18.2.tar.gz’ had non-zero exit status

RStanARM Version:

2.18.2

R Version:

3.5.1

Operating System:

Windows 10 10.0.15063

Start with the Windows build settings in makevars.win and the rstantools reference at R/rstan_package_skeleton.R line 155. Reproduce the failure with devtools::install_github("stan-dev/rstanarm") on Windows, then verify that the package and the related precompiled-model workflow build successfully with the supported compiler configuration.