Since commit ed0075bd (Use relative paths for custom command inputs,
2011-06-22) CMake generates full paths to source files in VS 10 project
files to avoid trouble with deep source/build tree paths.  However, the VS
10 IDE will not populate the source file property dialog for a file
referenced by full path.  Instead use a relative path when possible.  When
not possible produce a detailed warning explaining the problem and
suggesting use of shorter directory paths.