win,build: fix building from Windows SDK or VS console

Introduced in
libuv@2f6d4b4 and accidentally undone in
libuv@19c896d.

PR-URL: libuv#1415
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>

Author: saghul

vcbuild.bat

@@ -43,6 +43,9 @@ shift
 goto next-arg
 :args-done
 
+if defined WindowsSDKDir goto select-target
+if defined VCINSTALLDIR goto select-target
+
 @rem Look for Visual Studio 2017 only if explicitly requested.
 if "%target_env%" NEQ "vs2017" goto vs-set-2015
 echo Looking for Visual Studio 2017