On windows, maybe all platforms. The default BOOTDIR doesn't work when doing a
cd control/make; gnumake all

The hotspot build fails because it's not getting a BOOTDIR setting at all and then goes after a J:/ drive jdk which I suspect is in the hotspot/make/Makefile as the default.
Only works if you set ALT_BOOTDIR in your environment.
Problem is likely in hotspot/make/Makefile or control/make/hotspot-rules.gmk.

The control make needs to pass on it's default settings (when no ALT is used) to the hotspot makefile.