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

Introduced in
2f6d4b4b58 and accidentally undone in
19c896dd08.

PR-URL: https://github.com/libuv/libuv/pull/1415
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Refael Ackermann <refack@gmail.com>

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