bpo-33522: Enable CI builds on Visual Studio Team Services (#6865)

diff --git a/Tools/ssl/multissltests.py b/Tools/ssl/multissltests.py
index a51566c..2efc425 100755
--- a/Tools/ssl/multissltests.py
+++ b/Tools/ssl/multissltests.py
@@ -125,6 +125,11 @@
         "all and runs the test suite."
     )
 )
+parser.add_argument(
+    '--system',
+    default='',
+    help="Override the automatic system type detection."
+)
 
 
 class AbstractBuilder(object):
@@ -152,6 +157,7 @@
         # build directory (removed after install)
         self.build_dir = os.path.join(
             self.src_dir, self.build_template.format(version))
+        self.system = args.system
 
     def __str__(self):
         return "<{0.__class__.__name__} for {0.version}>".format(self)
@@ -255,9 +261,13 @@
         log.info("Running build in {}".format(self.build_dir))
         cwd = self.build_dir
         cmd = ["./config", "shared", "--prefix={}".format(self.install_dir)]
-        self._subprocess_call(cmd, cwd=cwd)
+        env = None
+        if self.system:
+            env = os.environ.copy()
+            env['SYSTEM'] = self.system
+        self._subprocess_call(cmd, cwd=cwd, env=env)
         # Old OpenSSL versions do not support parallel builds.
-        self._subprocess_call(["make", "-j1"], cwd=cwd)
+        self._subprocess_call(["make", "-j1"], cwd=cwd, env=env)
 
     def _make_install(self, remove=True):
         self._subprocess_call(