ease use of cmdstan makefiles

Summary:

This aims to simplify how users can use cmdstan with its makefiles.

Description:

Currently users have to change into the directory of cmdstan in order to build any Stan program like

cd path/to/cmdstan
make path/to/model

It would be much nicer if the user would not have to change into the directory of cmdstan, but can merely do this in any directory:

make -f path/to/cmdstan/makefile model

This currently does not work as the makefiles expect to be in the top-level directory of cmdstan. If the above would work, then users can define an alias like

alias stanmake=`make -f absolute/path/to/cmdstan/makefile`

and then people can just to

stanmake model

and that's it. Much simpler.

Reproducible Steps:

NA

Current Output:

NA

Expected Output:

NA

Additional Information:

Provide any additional information here.

Current Version:

v2.21.0

Start by locating the CmdStan makefiles and inspect assumptions that the working directory is the CmdStan top level. Verify the requested make -f path/to/cmdstan/makefile model invocation from another directory, including the existing model-building behavior; done means users can build without changing directories.