ocamlbuild: add an interface knob to decide whether we are in an ocamlbuild project

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15109 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
ocamlbuild: interpret tag-line before slurp, in case it contains "<foo>: traverse"

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15110 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#5851: ocamlbuild: hint when no _tags or myocamlbuild.ml are present

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15111 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#5406 ocamlbuild: "tag 'package' does not expect a parameter"

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15112 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
ocamlbuild: fix the -use-ocamlfind documentation (not the default)

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15113 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#6300: ocamlbuild -use-ocamlfind conflicts with -ocamlc

Note: there is a known issue with how -use-ocamlfind work: its
semantics is to change the compilation commands (ocamlc etc.) during
the option initialisation phase. This new change make it still take
into account changes to the command that would have been requested by
the user at option time (-ocamlc ...) or before (Before_option
dispatch in a plugin). But if you only activate use_ocamlfind after
the options (After_option dispatch), the command will not be changed,
but the "-package ..." ocamlfind-specific flags will still be passed
at compiler invocation.
In other words, use-ocamlfind must always be set *strictly* before
After_options (in a plugin or through the command-line), otherwise
weird stuff happens.

This glitch would be fixable in this commit (by moving
ocamlfind-command-initialization after After_options), but doing so
could have bad backward-compatibility consequences: current users may
very-well work-around PR#6300 by manually setting the command
(eg. Options.ocamlc := "ocamlfind ocamlcp") at After_options
dispatch. If we moved ocamlfind's initialization after After_options,
plugins using this manual workaround would stop doing what the users
expect (the ocaml command would become "ocamlfind ocamlfind ocamlcp"
in this example: not cool).

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15114 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
ocamlbuild: better provenance for syntax error messages

git-svn-id: http://caml.inria.fr/svn/ocaml/version/4.02@15115 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02