try to fix openmp on osx

setup.py

@@ -236,11 +236,14 @@ def build_extensions(self):
                 opts.append('/DVERSION_INFO=\\"%s\\"' % self.distribution.get_version())
 
             # setup OpenMP support
-            openmp_enabled, needs_gomp = detect_openmp(self.compiler)
+            openmp_enabled, additional_libs = detect_openmp(self.compiler)
             if openmp_enabled:
                 warnings.warn('enabled openmp')
-                omp_compiler_args = ['-fopenmp']
-                omp_libraries = ['-lgomp'] if needs_gomp else []
+                if sys.platform == 'darwin':
+                    omp_compiler_args = ['-fopenmp=libiomp5']
+                else:
+                    omp_compiler_args = ['-fopenmp']
+                omp_libraries = additional_libs
                 omp_defines = [('USE_OPENMP', None)]
             # debug
             dbg_flag = ['-g0' if not self.debug else '-g']

setup_util.py

@@ -90,17 +90,24 @@ def has_function(cc, funcname, headers):
 
 
 def detect_openmp(compiler):
+    from copy import deepcopy
+    compiler = deepcopy(compiler) # avoid side-effects
     with stdchannel_redirected(sys.stderr, os.devnull), \
          stdchannel_redirected(sys.stdout, os.devnull):
         has_openmp = has_function(compiler, 'omp_get_num_threads', headers='omp.h')
         print('[OpenMP] compiler {} has builtin support'.format(compiler))
-        needs_gomp = has_openmp
+        additional_libs = []
         if not has_openmp:
             print('[OpenMP] compiler {} needs library support'.format(compiler))
-            compiler.add_library('gomp')
+            if sys.platform == 'darwin':
+                compiler.add_library('iomp5')
+            elif sys.platform == 'linux2':
+                compiler.add_library('gomp')
             has_openmp = has_function(compiler, 'omp_get_num_threads', headers='omp.h')
-            needs_gomp = has_openmp
-        return has_openmp, needs_gomp
+            if has_openmp:
+                additional_libs = compiler.libraries[-1]
+                print('[OpenMP] added library {}'.format(additional_libs))
+        return has_openmp, additional_libs
 
 
 # has_flag and cpp_flag taken from https://github.com/pybind/python_example/blob/master/setup.py