Use -dumpfullversion if available in makefiles

[WardBrian]

Summary

We use -dumpversion to get the version of our C++ compiler, but this only reports the major version for newer GCCs.

See: https://stackoverflow.com/questions/45168516/gcc-7-1-1-on-fedora-26-dumpversion-now-only-includes-major-version-by-default

This works because gcc (and clang, from testing) ignore unknown -dump* flags

Tests

None

Side Effects

CXX_MINOR should be more consistently populated, making downstream usages better

Release notes

Checklist

• Math issue #(issue number)

• Copyright holder: (fill in copyright holder information)

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the 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/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested