CmdStan Guide needs better examples of compiler options

[mitzimorris]

Summary:

It would be good to add to CmdStan user guide a section about how to set different compiler options. Currently the user guide has only the example CXXFLAGS = -O2 Which is bad, as it is not with += and doesn't even remove -O3 as optimization level should be set with O=2 as dicussed in this thread

Current Version:

v2.29.0