wxWdgets is a cross-platform library that includes support for all the major windows versions, but I can't find anywhere in the wxWidgets documentation that says anything about the portability of a single MSW (windows) build of the wxWidgets library across different windows versions. Assuming my core app just uses vanilla C++ (and perhaps the least common denominator of win32 apis available on all platforms) except for wx functions, would linking statically against a single version of the wxWidgets library produce a binary that would run across Win2K, XP, Vista, and windows 7? Would that library need to be built on win2K (the "least featureful" platform?), or would building it on XP also work? Any hints or pointers would be appreciated!
You can build the application on any OS version you want, this doesn't affect the application compatibility with different Windows versions. You can even cross-compile Windows applications on another OS.
The important thing is the selection of API calls that the application references. Any functions linked at loadtime need to be present in the OS, while libraries or API functions that are delay loaded don't affect the compatibility, as long as the application handles the errors gracefully.
The stable wxWidgets version 2.8 is compatible with at least Windows 2000 and all later versions (you could even target Windows 9X if you use the non-Unicode build of the library), it delay loads all functions that are not available in Windows 2000 (for example theme support, introduced with XP). You can be sure that the application runs, but there may be things that don't work on Windows 2000 (for example transparent PNGs).
There may be additional minimum OS version requirements imposed by the development environment. Newer Visual C++ versions for example may require the C runtime to be installed on older OS versions, and there may be a minimum supported version. This however is independent of wxWidgets being used in the program or not.