Use "INNO" env var in Inno Setup script only if it's defined.

Use "INNO" env var in Inno Setup script only if it's defined.

Just use the sources directory itself if %INNO% is not defined. In fact, I
think we could always use the source directory, actually, but just in case
there was some reason to do it like this, keep using %INNO% if it is defined.

Also check that WXW_VER environment variable that we use is, in fact, defined.

FIX: Use relative path, not %WXWIN% in ISS script.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74559 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775