Bug 21594 – Writing object files should be read-then-write

It turns out that oftentimes the generated object file for a given .d file is identical to the preexisting one: * The build is triggered by a change in a dependent file that was not material to building this object file (most often) * Only unittests were changed and this is not a unittest build * Only documentation was changed In that case it's worth reading the existing object file (block by block) and comparing it against the object file to be written. * If the object files are different, proceed with writing as usual. * If the file are identical, do NOT overwrite the file. Do touch it to announce to tbe build system that the build has been done. This should be particularly gainful on SSD drives, which are much cheaper to read than to write. Comparison of files should be done block by block, not by slurping the entire object file. This is because once the headers are different all time spent reading the rest of the file is wasted.

*** This issue has been marked as a duplicate of issue 21593 ***