I have a a Win32 DLL project in VS2008, it is written in a handful of C modules.
Because I also want to be able to build outside VS2008, with no dependency on VS200
First of all, you shouldn't even need /Tc
if you're building it yourself - cl.exe uses file extension to determine the type, so .c files will be compiled as C by default, and .cpp and .cxx files as C++.
For VS projects, it works in exact same way, except that you can't override this behavior (or at least I do not know how).