[build] add support of Visual Studio 2017 and 2019 to the Visual C++ project file (#2891)

Author: Taritsyn

win/libsass.vcxproj

@@ -66,6 +66,12 @@
   <PropertyGroup Label="VS2015 toolset selection" Condition="'$(VisualStudioVersion)' == '14.0'">
     <PlatformToolset>v140</PlatformToolset>
   </PropertyGroup>
+  <PropertyGroup Label="VS2017 toolset selection" Condition="'$(VisualStudioVersion)' == '15.0'">
+    <PlatformToolset>v141</PlatformToolset>
+  </PropertyGroup>
+  <PropertyGroup Label="VS2019 toolset selection" Condition="'$(VisualStudioVersion)' == '16.0'">
+    <PlatformToolset>v142</PlatformToolset>
+  </PropertyGroup>
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'" Label="Configuration">
     <UseDebugLibraries>true</UseDebugLibraries>
   </PropertyGroup>