Add the CXX flag for only running rev tests with the test_ad

[rok-cesnovar]

Summary

This adds a CXX flag that can be used in Stan Math tests to not compile and run fwd & mix tests if the tests are using the test_ad utility.

I am primarily trying to add this so we can remove compiling & running fwd & mix tests with Windows and Rtools 3.5 (the tests that run with Github Actions). Particularly our mix tests are on the limit of what g++ 4.9.3 can handle, which is why we sometimes have to split tests files so that 4.9.3 can compile them.

I doubt anyone is using g++ 4.9.3 with fwd and mix. It is still used with Stan, but we only need rev tests to check compatibility there.

There is no Math issue for this PR. I can make one if needed.

Tests

This will decrease our testing with Windows Rtools 3.5 (g++ 4.9.3). Given that we using g++ 4.9.3 outside of the use in Stan is not really recommended, I think this should be fine.

Side Effects

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Checklist

• Copyright holder: Rok Češnovar

  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/)

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	3.56	3.62	0.98	-1.58% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.99	-0.75% slower
eight_schools/eight_schools.stan	0.09	0.09	0.96	-4.42% slower
gp_regr/gp_regr.stan	0.14	0.14	1.0	0.14% faster
irt_2pl/irt_2pl.stan	5.69	5.74	0.99	-0.75% slower
performance.compilation	93.67	91.13	1.03	2.71% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.05	8.05	1.0	0.03% faster
pkpd/one_comp_mm_elim_abs.stan	31.84	33.03	0.96	-3.76% slower
sir/sir.stan	118.96	121.22	0.98	-1.9% slower
gp_regr/gen_gp_data.stan	0.03	0.03	1.0	0.31% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	3.01	2.98	1.01	0.85% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.38	0.41	0.93	-8.06% slower
arK/arK.stan	2.08	2.09	1.0	-0.49% slower
arma/arma.stan	0.23	0.24	0.97	-3.16% slower
garch/garch.stan	0.58	0.59	0.98	-2.01% slower
Mean result: 0.985595282792

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Jenkins Console Log
Blue Ocean
Commit hash: 90969fc

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Machine information

ProductName: Mac OS X ProductVersion: 10.11.6 BuildVersion: 15G22010

CPU:
Intel(R) Xeon(R) CPU E5-1680 v2 @ 3.00GHz

G++:
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

Clang:
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin15.6.0
Thread model: posix

[rok-cesnovar]

This is ready for review. It cuts about an hour from the Windows RTools 3.5 mix tests.

[SteveBronder]

lgtm! As long as it's only used for windows testing I think it's fine. Though I think once we bump up the compiler version we test on we may want to think about turning this back off

[rok-cesnovar]

Yes, once we bump, this should go away!