One of the ICU release (BRS) items is to update the pool.res bundles (which contain the union of resource bundle key strings of all bundles in their trees). This is easy on Windows where nmake passes all of the (modified?) dependency files to the command via $(<F). Not so with gmake which invokes the command for each of the dependencies.
For ICU 4.6 I temporarily changed the data/Makefile in r28947 to use $(RES_SRC) etc. rather than $(<F). In a single-threaded make run (no -jN) this works fine. However, in a multi-threaded make run it invokes the command for each dependency, and each invocation reads all dependencies, taking several minutes to complete. I reverted this with r28957.
I would like to explore some other way of getting genrb invoked once with all modified dependencies, but it should not see all dependencies if only one or two were modified.
If all else fails, there should be a new Linux Makefile target (run only at release time) for generating new pool.res files.