When an idl file is updated, files like HandlerData.h are generated as
part of the midl target (eg: midl_done). However, Make may have already
stat'd HandlerData.h and cached its timestamp. Although there is a
dependency from HandlerData.h on midl_done, there is no recipe. As such,
Make assumes that HandlerData.h hasn't actually changed, and uses the
cached value of the timestamp when determining if it should install the
file into dist/include. If the cached value is older, make may not
trigger the install rule, leaving the old header in place and breaking
the build.
MozReview-Commit-ID: 9rdtXIt8mXC
44d83112fe