Fix for problem with compiling the PYTHON interface in Mac OS 10.14 and higher.

Sets the deployment target to 10.9 when compiling on version 10.9 and above.
The fix is taken from https://github.com/pandas-dev/pandas/pull/24274/files

GB: removed whitespace change and merged. Closes #445

Thanks @valsson

python/setup.py

@@ -29,6 +29,13 @@ import os
 import os.path
 import sys
 from shutil import copyfile
+import platform
+from distutils.sysconfig import get_config_var
+from distutils.version import LooseVersion
+
+
+def is_platform_mac():
+    return sys.platform == 'darwin'
 
 if os.getenv("plumed_macports") is not None:
     copyfile("../VERSION","VERSION")
@@ -52,6 +59,19 @@ if defaultkernel is not None:
     extra_compile_args.append("-D__PLUMED_DEFAULT_KERNEL=" + os.path.abspath(defaultkernel))
     print( "Hardcoded PLUMED_KERNEL " + os.path.abspath(defaultkernel))
 
+# Fixes problem with compiling the PYTHON interface in Mac OS 10.14 and higher.
+# Sets the deployment target to 10.9 when compiling on version 10.9 and above,
+# overriding distutils behaviour to target the Mac OS version python was built for.
+# This can be overridden by setting MACOSX_DEPLOYMENT_TARGET before compiling the
+# python interface.
+# This fix is taken from https://github.com/pandas-dev/pandas/pull/24274/files
+if is_platform_mac():
+    if 'MACOSX_DEPLOYMENT_TARGET' not in os.environ:
+        current_system = LooseVersion(platform.mac_ver()[0])
+        python_target = LooseVersion(get_config_var('MACOSX_DEPLOYMENT_TARGET'))
+        if python_target < '10.9' and current_system >= '10.9':
+            os.environ['MACOSX_DEPLOYMENT_TARGET'] = '10.9'
+
 def readme():
     with open('README.rst') as f:
         return f.read()