Index: lit/helper/build.py
===================================================================
--- lit/helper/build.py
+++ lit/helper/build.py
@@ -512,7 +512,7 @@
         defaultenv = {}
         if sys.platform == 'win32':
             defaultenv = { x : os.environ[x] for x in
-                          ['SystemDrive', 'SystemRoot', 'TMP', 'TEMP'] }
+                          ['SystemDrive', 'SystemRoot', 'TMP', 'TEMP', 'VCINSTALLDIR'] }
             # The directory to mspdbcore.dll needs to be in PATH, but this is
             # always in the native toolchain path, not the cross-toolchain
             # path.  So, for example, if we're using HostX64\x86 then we need