bpo-42504: Ensure that get_config_var('MACOSX_DEPLOYMENT_TARGET') is a string (GH-24341)

* bpo-42504: Ensure that get_config_var('MACOSX_DEPLOYMENT_TARGET') is a string
diff --git a/Lib/sysconfig.py b/Lib/sysconfig.py
index c1aaf79..507c51f 100644
--- a/Lib/sysconfig.py
+++ b/Lib/sysconfig.py
@@ -18,6 +18,11 @@
     'parse_config_h',
 ]
 
+# Keys for get_config_var() that are never converted to Python integers.
+_ALWAYS_STR = {
+    'MACOSX_DEPLOYMENT_TARGET',
+}
+
 _INSTALL_SCHEMES = {
     'posix_prefix': {
         'stdlib': '{installed_base}/{platlibdir}/python{py_version_short}',
@@ -252,6 +257,9 @@ def _parse_makefile(filename, vars=None):
                 notdone[n] = v
             else:
                 try:
+                    if n in _ALWAYS_STR:
+                        raise ValueError
+
                     v = int(v)
                 except ValueError:
                     # insert literal `$'
@@ -310,6 +318,8 @@ def _parse_makefile(filename, vars=None):
                         notdone[name] = value
                     else:
                         try:
+                            if name in _ALWAYS_STR:
+                                raise ValueError
                             value = int(value)
                         except ValueError:
                             done[name] = value.strip()
@@ -472,6 +482,8 @@ def parse_config_h(fp, vars=None):
         if m:
             n, v = m.group(1, 2)
             try:
+                if n in _ALWAYS_STR:
+                    raise ValueError
                 v = int(v)
             except ValueError:
                 pass