Support for MSVC

[dmilos]

1. In this pull request it is quick implementation for MSVC
2. This quick fix is not intended for merge in to main branch
3. Intention is to show how, what and where fix might be to support MSVC

After applying this patch and build using VS this will produce next files:

$\release\bin\jsonnet.dll
$\release\bin\jsonnet.exe
$\release\lib\jsonnet.lib
$\release\bin\jsonnet++.dll
$\release\lib\jsonnet++.lib
$\release\bin\jsonnetfmt.exe
$\release\include\libjsonnet.h
$\release\include\libjsonnet_fmt.h
$\release\include\libjsonnet++.h

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.