build: skip vcvarsall if already set up

Without this, running vcbuild repeatedly for some time will cause errors with msbuild saying the command line is too long (due to concatenation happening in vcvarsall).

Author: mscdex

vcbuild.bat

@@ -41,6 +41,8 @@ shift
 goto next-arg
 :args-done
 
+if defined VCINSTALLDIR goto select-target
+
 @rem Look for Visual Studio 2013
 if not defined VS120COMNTOOLS goto vc-set-2012
 if not exist "%VS120COMNTOOLS%\..\..\vc\vcvarsall.bat" goto vc-set-2012