enable config.cache per default

It speeds up configuration considerably.
  If a package breaks due to wrong cache-entries, then the broken
  other package has to be fixed, not the innocent bystander!

  Put short: There is absolutely _no_ need to ever turn the cache off
  unless you hack on autotools itself and goof.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>