New ${checkpoint} command to create an empty "checkpoint" file, to store the last modified date of a make target
modified tools/rbuild/backend/mingw/rules.mak
Rules that output more than one file at once don't mix well with parallel builds: serialize dependency checks through the use of checkpoint files. Fixes intermittent widl issues in highly parallel builds
Bonus internal rbuild function that I had to commit one day or another
See issue #4994 for more details.
svn path=/trunk/; revision=44594