check for camlp4 presence and version in configure

fixes issue #431
Do not remove the lock when it has not been locked
Merge branch 'issue/431' of git://github.com/oandrieu/opam
Update generated configure file
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