Avoiding unnecessary rebuilds with Automake

The attached patch:

  - adds a --no-touch option, which prevents all files (not just the
    public header) from being touched unless they have changed
  - while here, checks if a file has changed without cmp(1)
  - removes the timestamp from .c comment headers

It helps avoiding unnecessary rebuilds when GOB is used from Automake,
thanks to the following trick (the manual page should be updated):

foo_SOURCES = foo.gob foo.gob.stamp foo.c foo.h foo-private.h
BUILT_SOURCES = foo.gob.stamp
MAINTAINERCLEANFILES = foo.gob.stamp

%.gob.stamp: %.gob
        @GOB2@ --no-touch $<
        @touch $@

-- 
Jean-Yves Lefort
jylefort_at_brutele.be
http://lefort.be.eu.org/