build: add MSVS 2015 support

PR-URL: nodejs/node#2036
Reviewed-By: Alexis Campailla <alexis@janeasystems.com>

vcbuild.bat

@@ -88,6 +88,18 @@ if defined NIGHTLY set TAG=nightly-%NIGHTLY%
 
 @rem Set environment for msbuild
 
+@rem Look for Visual Studio 2015
+if not defined VS140COMNTOOLS goto vc-set-2013
+if not exist "%VS140COMNTOOLS%\..\..\vc\vcvarsall.bat" goto vc-set-2013
+if "%VCVARS_VER%" NEQ "140" (
+  call "%VS140COMNTOOLS%\..\..\vc\vcvarsall.bat"
+  SET VCVARS_VER=140
+)
+if not defined VCINSTALLDIR goto vc-set-2013
+set GYP_MSVS_VERSION=2015
+goto msbuild-found
+
+:vc-set-2013
 @rem Look for Visual Studio 2013
 if not defined VS120COMNTOOLS goto msbuild-not-found
 if not exist "%VS120COMNTOOLS%\..\..\vc\vcvarsall.bat" goto msbuild-not-found