[GUILT 1/5] Fix generation of Documentation/usage-%.txt.

The old rule worked, most of the time, but had several issues:

 - It depended on the corresponding guilt-*.txt file, but the usage.sh
   script actually reads ../guilt-foo.

 - Actually, each usage-%.txt depended on all guilt-*.txt files, so
   make had to do more work than necessary if a single file was
   altered.

 - The construct broke parallel make, which would spawn several
   usage.sh at once.  This leads to unnecessary work, and could
   potentially result in broken usage files if the "echo some_string >
   some_file" construct used by usage.sh isn't atomic.

Fixed by letting the usage.sh script update a single file, and writing a proper implicit make rule. This makes parallel make work a lot better.

There is a small downside, though, as usage.sh will now be run once for each command (if everything is regenerated). I think it is worth to pay that price to get the correctness. This command is still very fast compared to the docbook processing.

Signed-off-by: Per Cederqvist <cederp@opera.com>
---
 Documentation/Makefile | 4 ++--
 Documentation/usage.sh | 8 +++-----
 2 files changed, 5 insertions(+), 7 deletions(-)