autoconf: More helpful error message to indicate how to possibly fix the camlp4 missing problem
remove caml4boot from the test, as its not used and sometimes (on Debian?) not installed.
regenerate autoconf from 1465af9f99edd0533416ca397e7a7885488b769a
Merge pull request #433 from avsm/master

autoconf fixes for 3.12.1