build: vcbuild fix "The input line is too long."

vcbuild.bat is calling vcvars.bat, which doesn't detect if the environment
has already been set. This causes repeated entries to be added to the PATH,
which after a few invocations will lead to an error:
The input line is too long.

vcbuild.bat

@@ -108,7 +108,10 @@ if defined nobuild goto sign
 @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
+if "%VCVARS_VER%" NEQ "120" (
   call "%VS120COMNTOOLS%\..\..\vc\vcvarsall.bat"
+  SET VCVARS_VER=120
+)
 if not defined VCINSTALLDIR goto msbuild-not-found
 set GYP_MSVS_VERSION=2013
 goto msbuild-found
@@ -117,7 +120,10 @@ goto msbuild-found
 @rem Look for Visual Studio 2012
 if not defined VS110COMNTOOLS goto vc-set-2010
 if not exist "%VS110COMNTOOLS%\..\..\vc\vcvarsall.bat" goto vc-set-2010
+if "%VCVARS_VER%" NEQ "110" (
   call "%VS110COMNTOOLS%\..\..\vc\vcvarsall.bat"
+  SET VCVARS_VER=110
+)
 if not defined VCINSTALLDIR goto msbuild-not-found
 set GYP_MSVS_VERSION=2012
 goto msbuild-found
@@ -125,7 +131,10 @@ goto msbuild-found
 :vc-set-2010
 if not defined VS100COMNTOOLS goto msbuild-not-found
 if not exist "%VS100COMNTOOLS%\..\..\vc\vcvarsall.bat" goto msbuild-not-found
+if "%VCVARS_VER%" NEQ "100" (
   call "%VS100COMNTOOLS%\..\..\vc\vcvarsall.bat"
+  SET VCVARS_VER=100
+)
 if not defined VCINSTALLDIR goto msbuild-not-found
 goto msbuild-found