some changes to the autoconf support
- added config.cache to "make clean". We get two many bug reports from
people builing on multiple architectures who don't remove config.cache
between compiles. Hopefully a future version of autoconf will do
this automatically
- removed the targets that run autoconf and autoheader. There is far
too great a risk of a enduser getting a error "autoheader not found"
if the date on a file is changed. Samba developers will have to run
autoconf and autoheader manually (as before)