Fix buildoutput pathvars

[tvandijck]

It turns out, that Visual Studio really don't like having most of the pathVars in the <Outputs> property of a custom build command....

this just doesn't work. The command does the right thing, but the tracking of the outputs goes wrong, and visual studio manages to actually create a $(SolutionDir) folder on disk too.

    <CustomBuild Include="..\..\..\packages\tag_edit\include\tag_edit\tag_edit.tag">
      <FileType>Document</FileType>
      <Command>tagc --cpp --types --schema --out=$(SolutionDir)/generated_tag/tag_edit %(FullPath)</Command>
      <Outputs>$(SolutionDir)/generated_tag/tag_edit/tag_edit.h;$(SolutionDir)/generated_tag/tag_edit/tag_edit.cpp</Outputs>
      <Message>Compiling %(FullPath)</Message>
    </CustomBuild>

So instead of making pathVars the default for buildoutputs and buildinputs, we turn it off, and only turn it on through an override in the environment where appropriate.