ENH: Added computation of object file names that are almost always short enough to not exceed the filesystem path length limitation.  This is useful when a source file from outside the tree is referenced with a long full path.  The object file name previously would contain the entire path which when combined with the build output directory could exceed the filesystem limit.  Now CMake recognizes this case and replaces enough of the beginning of the full path to the source file with an md5sum of the replaced portion to make the name fit on disk.  This addresses bug#4520.