Fix Makefile targets that generate several files at once

Fix Makefile targets that generate several files at once

In a few places in the tree the Makefiles have constructs like this:
   one_file another_file:
       $(COMMAND_WHICH_GENERATES_BOTH_AT_ONCE)
This is wrong, because make will run _two copies_ of the same command
at once.  This generally causes races and hard-to-reproduce build
failures.

Notably, `make -j4' at the top level will build stubdom libxc twice
simultaneously!

In this patch we replace the occurrences of this construct with the
correct idiom:
   one_file: another_file
   another_file:
       $(COMMAND_WHICH_GENERATES_BOTH_AT_ONCE)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>