Simplify initial auto.conf setup

Instead of creating an empty initial auto.conf and immediately loading
it, look for a missing auto.conf in _load_old_vals() and treat that the
same as an empty auto.conf (by returning after clearing the old values).

The initial auto.conf will then be written as normal by
_write_old_vals().

Also flesh out the sync_deps() docstring a bit:

 - Make it clear that sync_deps() should be run at each build

 - Be more explicit about what it means for a symbol to have changed
   (different output in autoconf.h)

0 files changed, 0 insertions, 0 deletions