Do not print `***' when compiling unless we fall over

Do not print `***' when compiling unless we fall over

This has finally bugged me enough to do something about it:

make prints `***' when it stops due to an error.  With make -j, and/or
with automated build systems, this error marker can be in the middle
of a large logfile.  It is useful to be able to search for it.

Therefore change occurrences of `*' to `=' for information and warning
messages.  `***' is reserved for cases where the build stops.

A corresponding change is being applied to qemu-xen-unstable.

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