As far as CRAN is concerned, there are two flavors of R on Solaris: one that is built using the Solaris Studio compiler, and one that is built using the GNU/gcc toolchain. The latter is far more up-to-date, but if your package requires it, then your DESCRIPTION file must declare that with the line
SystemRequirements: GNU make.
These instructions are for configuring, building, and installing R-devel using the GNU/gcc toolchain (only).
You'll need VMWare Fusion on Mac, or VMWare Workstation (?) on Windows/Linux.
Get Solaris VM
Download the Solaris VM provided by Jeroen Ooms: https://github.com/jeroen/solarisvm