Update OSX build instructions

[lenguyenthedat]

Keeping things simpler for OS X users:

• tested on macOS Sierra 10.12.6
• brew install gcc is now at v 0.7.2,
• --without-multilib isn't an option anymore:

$ brew info gcc
gcc: stable 7.2.0 (bottled), HEAD
GNU compiler collection
https://gcc.gnu.org/
/usr/local/Cellar/gcc/7.2.0 (1,487 files, 284MB) *
  Poured from bottle on 2017-10-11 at 23:04:26
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/gcc.rb
==> Dependencies
Required: gmp ✔, libmpc ✔, mpfr ✔, isl ✔
==> Options
--with-jit
	Build just-in-time compiler
--with-nls
	Build with native language support (localization)
--HEAD
	Install HEAD version

(it's actually not needed in our case as there is no incompatibility between OpenMP and multilib support Homebrew/homebrew-core#16751)

[codecov-io]

Codecov Report

Merging #2784 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #2784      +/-   ##
============================================
- Coverage     43.16%   43.08%   -0.08%     
- Complexity      192      200       +8     
============================================
  Files           150      151       +1     
  Lines         11368    11519     +151     
  Branches       1148     1167      +19     
============================================
+ Hits           4907     4963      +56     
- Misses         6135     6225      +90     
- Partials        326      331       +5

Impacted Files	Coverage Δ	Complexity Δ
include/xgboost/tree_model.h	33.08% <0%> (-7.6%)	0% <0%> (ø)
src/predictor/cpu_predictor.cc	69.62% <0%> (-2.64%)	0% <0%> (ø)
src/learner.cc	25.92% <0%> (-0.36%)	0% <0%> (ø)
.../scala/ml/dmlc/xgboost4j/scala/spark/XGBoost.scala	73.02% <0%> (-0.19%)	0% <0%> (ø)
python-package/xgboost/core.py	80.97% <0%> (-0.13%)	0% <0%> (ø)
src/c_api/c_api.cc	17.92% <0%> (-0.04%)	0% <0%> (ø)
tests/cpp/predictor/test_cpu_predictor.cc	100% <0%> (ø)	0% <0%> (ø)	⬇️
include/xgboost/gbm.h	14.28% <0%> (ø)	0% <0%> (ø)	⬇️
include/xgboost/predictor.h	75% <0%> (ø)	0% <0%> (ø)	⬇️
src/common/row_set.h	0% <0%> (ø)	0% <0%> (ø)	⬇️
... and 3 more

---

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[lenguyenthedat]

Any chance this get merged? The original instructions are duplicative and quite confusing sometimes.

[superbobry]

Could you mention that Clang is OK if OMP support is not needed?

[lenguyenthedat]

@superbobry Done.

Also, I'm wondering if we should include an instruction for pip install method.
Currently if you want to use pip install xgboost, you can only do it with gcc5. More info: https://medium.com/@lenguyenthedat/installing-xgboost-on-os-x-1f63c1ed042

Lots of people have been facing that problem too: https://github.com/dmlc/xgboost/issues?utf8=%E2%9C%93&q=%22failed%20with%20error%20code%201%22

[superbobry]

Good idea, please go ahead.

[lenguyenthedat]

Done 😄

[lenguyenthedat]

Shall we merge? 😄

[superbobry]

Thanks @lenguyenthedat!