I did finally just redo the make build to see what would happen and viola, none of the warnings or single error occurred on the second pass.
I can't say for sure, but one possibility is this: Unless you "make clean", the results of the previous compilation (that is, the output object files) will still be hanging around. Because of the way in which the make system looks at file timestamps to determine whether or not a file needs to be recompiled or not, it is just possible that the offending files were skipped for recompilation because there was already a compiled object and the source files had not been changed. If they weren't recompiled, then warnings would not appear.
This warrants a little more explanation. Since projects can take a long time to build, it is generally the case that only parts of the project that need recompilation will in fact be recompiled. The Make system deals with this by looking at each source file, and any files that they depend upon, to see if they have changed since the last make. If they have not, and there is already a suitable output file for the source, then recompilation is skipped. It really saves a lot of time for the developers! The make file output should indicate if this is the case.
Errors are another matter. Depending upon how the compiler options are set, Errors can terminate a compile/build. Another make should trip up on the same error. Typically, but not always, the compiler options are set to trap most (but not necessarily all) errors.
Hope this helps