msvs: set $(TargetExt) when the "product_extension" is set

msvs: set $(TargetExt) when the "product_extension" is set

For node.js native modules, we set the "product_extension" to "node", so that
the resulting loadable_module has a ".node" file extension. On Windows, doing
this results in a msbuild warning along the lines of:

  warning MSB8012: TargetExt(.dll) does not match the Linker's OutputFile
  property value (.node). This may cause your project to build incorrectly.
  To correct this, please make sure that $(OutDir), $(TargetName) and
  $(TargetExt) property values match the value specified in %(Link.OutputFile).

This patch properly sets the $(TargetExt) variable to fix the warning.

BUG=

[M-A Ruel, 2012-11-07 13:52:00]

1. You can't use the commit queue for GYP.
2. You need to click "Publish+Mail Comments" to request a code review.

[tootallnate, 2012-11-08 20:04:26]

On 2012/11/07 13:52:00, Marc-Antoine Ruel wrote:
> 1. You can't use the commit queue for GYP.
> 2. You need to click "Publish+Mail Comments" to request a code review.

Thanks for the reply. Can you clarify what you mean in #1?

[bradn, 2012-11-16 22:55:45]

Adding scott.
A test of the changed behavior would be idea.