fix installation bug with openmp on mac

Author: jlyang1990

README.md

@@ -50,6 +50,12 @@ cd fasttreeshap
 pip install .
 ```
 
+Installation troubleshooting:
+* On Macbook, if an error message `ld: library not found for -lomp` pops up, run the following command line before installation ([Reference](https://iscinumpy.gitlab.io/post/omp-on-high-sierra)):
+```sh
+brew install libomp
+```
+
 ## Usage
 
 The following screenshot shows a typical use case of FastTreeSHAP on [Census Income Data](https://archive.ics.uci.edu/ml/datasets/census+income). Note that the usage of FastTreeSHAP is exactly the same as the usage of [SHAP](https://github.com/slundberg/shap), except for three additional arguments in the class `TreeExplainer`: `algorithm`, `n_jobs`, and `shortcut`.

setup.py

@@ -80,11 +80,10 @@ def run_setup(with_binary, with_openmp, test_xgboost, test_lightgbm, test_catboo
             compile_args.append('/MD')
         if with_openmp:
             if sys.platform == "darwin":
-                compile_args.append('-Xpreprocessor')
-                link_args.append('-lomp')
+                compile_args += ['-Xpreprocessor', '-fopenmp', '-lomp']
             else:
+                compile_args.append('-fopenmp')
                 link_args.append('-fopenmp')
-            compile_args.append('-fopenmp')
 
         ext_modules.append(
             Extension('fasttreeshap._cext', sources=['fasttreeshap/cext/_cext.cc'],