Cleanup the use of find in makefile

[rok-cesnovar]

Summary

This PR fixes the Stan Math side of stan-dev/cmdstan#786

The issue is that our makefiles rely on the gnu find command that is not available on Windows.

This PR introduces a makefile subroutine that calls dir on Windows and find on Unix systems.

ifeq ($(OS),Windows_NT)
findfiles=$(shell cmd.exe /C "dir /b /s $(subst /,,$1)$2 2>NUL")
else
findfiles=$(shell find $1 -type f -name $2)
endif

The 2>NUL is there because if you search for files that dont exist, dir will output a File not found error. So if you called make clean twice in a row it would output a bunch of File not founds.

This PR introduces a findfiles function that is a recursive version of the makefiles wildcard function.
findfiles=$(foreach d,$(wildcard $(addsuffix *,$(1))),$(call findfiles,$(d)/,$(2)) $(filter $(subst *,%,$(2)),$(d)))

This PR also excludes the generaton of distribution tests on Windows. This means that make generate-tests will not be available on Windows by default. This functionality is currently not working on Windows and just produces a bunch of warnings. Making this functionality available on Windows should be addressed separately. That requires a larger overhaul and since its a functionality aimed at developers its not something that needs to be addressed before the next release. The unnecessary warnings seen by users compiling models is a priority in my opinion.

Tests

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Side Effects

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• 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]

(stat_comp_benchmarks/benchmarks/gp_pois_regr/gp_pois_regr.stan, 1.0)
(stat_comp_benchmarks/benchmarks/low_dim_corr_gauss/low_dim_corr_gauss.stan, 0.98)
(stat_comp_benchmarks/benchmarks/irt_2pl/irt_2pl.stan, 0.99)
(stat_comp_benchmarks/benchmarks/pkpd/one_comp_mm_elim_abs.stan, 0.97)
(stat_comp_benchmarks/benchmarks/eight_schools/eight_schools.stan, 0.99)
(stat_comp_benchmarks/benchmarks/gp_regr/gp_regr.stan, 1.02)
(stat_comp_benchmarks/benchmarks/arK/arK.stan, 0.91)
(performance.compilation, 1.01)
(stat_comp_benchmarks/benchmarks/low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan, 0.99)
(stat_comp_benchmarks/benchmarks/low_dim_gauss_mix/low_dim_gauss_mix.stan, 0.99)
(stat_comp_benchmarks/benchmarks/sir/sir.stan, 1.07)
(stat_comp_benchmarks/benchmarks/pkpd/sim_one_comp_mm_elim_abs.stan, 1.0)
(stat_comp_benchmarks/benchmarks/garch/garch.stan, 1.01)
(stat_comp_benchmarks/benchmarks/gp_regr/gen_gp_data.stan, 0.99)
(stat_comp_benchmarks/benchmarks/arma/arma.stan, 1.01)
Result: 0.99512462713
Commit hash: fb7224b

[syclik]

That's slick!