Some of these steps are going to get a bit easier with the next version of rstantools. If you add
SystemRequirements: GNU make
to DESCRIPTION, then src/Makevars can use syntax that works everywhere but Solaris and even works on Solaris if gmake is installed. So, you can do things like
SOURCES = $(wildcard stan_files/*.stan)
and not have to explicitly list the Stan programs that you want to compile.