Creation of icudata.rc can fail in parallel build mode
I build ICU on Windows with MSYS/MSVC, using GNU make and the MinGW environment that comes with Git for Windows which is sufficient for the build.
Running a build today with `make -j8` I got an error trying to create icudata.rc:
Looking at the logs I can see that the command:
which will create that directory, was run in parallel with the above rc generation, so it seems like the directory was not created first.