Makefiles: Add $(strip) to findfiles function

[rok-cesnovar]

Summary

The work in #1784 needs an additional patch. This adds $(strip to remove whitespaces that occur on unix systems.

Tests

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Side Effects

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Release notes

Replace this text with a short note on what will change if this pull request is merged in which case this will be included in the release notes.

Checklist

• Math issue #(issue number)

• 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/)

• 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

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
gp_pois_regr/gp_pois_regr.stan	5.02	4.9	1.02	2.35% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.02	0.02	0.97	-2.73% slower
eight_schools/eight_schools.stan	0.1	0.09	1.04	4.19% faster
gp_regr/gp_regr.stan	0.22	0.22	0.99	-0.79% slower
irt_2pl/irt_2pl.stan	6.45	6.44	1.0	0.22% faster
performance.compilation	87.79	86.68	1.01	1.27% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	7.51	7.57	0.99	-0.8% slower
pkpd/one_comp_mm_elim_abs.stan	20.66	21.42	0.96	-3.7% slower
sir/sir.stan	91.67	90.93	1.01	0.81% faster
gp_regr/gen_gp_data.stan	0.05	0.05	0.97	-2.7% slower
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.96	2.95	1.0	0.33% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.31	0.31	0.99	-1.16% slower
arK/arK.stan	1.74	1.74	1.0	-0.18% slower
arma/arma.stan	0.66	0.66	1.0	0.21% faster
garch/garch.stan	0.52	0.51	1.01	0.54% faster
Mean result: 0.998950263977

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Jenkins Console Log
Blue Ocean
Commit hash: ce60882

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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

[SteveBronder]

I'm fine with approving if @mitzimorris doesn't have time to look at it.

One quick santify check, if this is linux specific should it be defined per system or will strip work everywhere?

[rok-cesnovar]

$Strip is a make command and works on all platforms.

[SteveBronder]

cool lgtm then!