As it turns out, I have been researching this issue recently for a C++ project I am working on (some related research for a smart pointer implementation led me to run across Boost and start paying attention to what you' all are up to :-). Anyway, one of the key goals for the project is cross-platform (NT and various Unix/Linux flavors) and cross-compiler (probably g++ and vcc) compatibility. Of course, we also want a fully automated build and test system so that we can rapidly detect and fix any changes which create porting issues. Unfortunately, none of this is really easy or "lightweight".

So far, it looks like the best answer incorporates 2 "tools". For the build part, use IMake to simplify makefile creation and factor out vendor and platform specific configurations. In a nutshell, Imake uses macros and template files to allow you to write one file (the IMakefile) and generate Makefiles for all the variations. You can find info on Imake at http://www.primate.wisc.edu/software/imake-stuff/. I have used Imake on a rather large C++ project (more than 200 people), and it worked quite nicely once it was set up. The other main alternative I have seen is to just maintain by hand Makefiles for every platform / compiler. This is what seems to have been done on ACE, and I hear it makes it really difficult to add files, remove files, reorganize directories, etc.

The other part of the equation is to automate testing using CppUnit from http://www.xprogramming.com/. CPPUnit is derived from work done by Kent Beck on testing frameworks which spawned. Of course CPPUnit is already fractured along compiler and platform lines as you can see from the download page. However, if you dump the GUI, the framework is trivial to port. Anyway, the idea is that a single command can test everything and report the results in the form of pass/fail statistics and diagnostic pointers for any failures. Again, as an example, while ACE doesn't use CppUnit, it does have a consistently written set of tests which allow rapid validation.