Allow providing external dependencies

[andrjohns]

Submission Checklist

• [x] Run unit tests: ./runTests.py src/test/unit
• [x] Run cpplint: make cpplint
• [x] Declare copyright holder and open-source license: see below

Summary

This PR follows the work in the Math library to allow users to provide external dependencies for Stan (instead of the bundled rapidjson and stan_math).

The PR introduces the following makeflags:

• MATH_INC
• RAPIDJSON_INC

Which can be used to provide paths to external dependencies

Intended Effect

Allow for increased portability and future options for packaging with homebrew and linux repos

How to Verify

Install rapidjson:

Homebrew

brew install rapidjson

Debian/Ubuntu

sudo apt install rapidjson-dev

Add to make/local:

Homebrew

RAPIDJSON_INC = /opt/homebrew/Cellar/rapidjson/1.1.0/include

Debian/Ubuntu

RAPIDJSON_INC = /usr/include/

Side Effects

N/A

Documentation

N/A

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[syclik]

Maybe what the repo needs are better examples...