make: using SUNDIALS_TARGETS instead of LIBSUNDIALS

[WardBrian]

Submisison Checklist

• Run tests: ./runCmdStanTests.py src/test
• Declare copyright holder and open-source license: see below

Summary:

Follow on to stan-dev/stan#3162 and stan-dev/math#2861, this change is for consistency and to allow users to use an external Sundials more easily.

After this we can clean this up in the Math makefiles: https://github.com/stan-dev/math/blob/98117fa4616d50d9544fed882c5a5c2b9803cf85/make/libraries#L84

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):

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/)

[rok-cesnovar]

Assuming tests pass, this is good to go.