Issue #9516: avoid errors in sysconfig when MACOSX_DEPLOYMENT_TARGET is set in shell.

Without this patch python will fail to start properly when the environment
variable MACOSX_DEPLOYMENT_TARGET is set on MacOSX and has a value that is
not compatible with the value during Python's build. This is caused by code
in sysconfig that was only meant to be used in disutils.
diff --git a/Lib/distutils/util.py b/Lib/distutils/util.py
index ce3cd6c..d6f89d6 100644
--- a/Lib/distutils/util.py
+++ b/Lib/distutils/util.py
@@ -96,9 +96,7 @@
         from distutils.sysconfig import get_config_vars
         cfgvars = get_config_vars()
 
-        macver = os.environ.get('MACOSX_DEPLOYMENT_TARGET')
-        if not macver:
-            macver = cfgvars.get('MACOSX_DEPLOYMENT_TARGET')
+        macver = cfgvars.get('MACOSX_DEPLOYMENT_TARGET')
 
         if 1:
             # Always calculate the release of the running machine,