Create a new file, output.c, and collect functions that generate output there.
We introduce a new global context specifying where output should go (to stdout
or to a sync file), and the lowest level output generator chooses where to
write output based on that context.
This allows us to set the context globally, and all operations that write
output (including functions like $(info ...) etc.) will use it.
Removed the "--trace=dir" capability. It was too confusing. If you have
directory tracking enabled then output sync will print the enter/leave message
for each synchronized block. If you don't want that, disable directory
tracking.
On configure-enabled systems, configure will detect Guile installed
(using pkg-config, which is how GNU Guile is distributed) and enable
it if so.
On all non-configure-enabled systems, currently, the default is for
Guile support to be disabled.
Incorporate "order-only" prerequisites patch. Wrote a test for it.
The test shows what might be a bug in the code; I need to look at it
more closely (anyway it doesn't behave as I expected). Also I haven't
done the docs yet.
