build: make CAMLOPT_BIN default to "no-opt-compiler-available".
    (Patch by Adrien Nader!)
    
    Jacques Garrigue has reported that the testsuite would try to run with a
    non-existing opt compiler and that the error wasn't clear.
    
    Now, instead of trying to run:
      -c -I some_dir/ foo.ml
    anything that tries to run ocamlopt before it's available will run:
      no-opt-compiler-available -c -I some_dir/ foo.ml
    which should hopefully be clearer.
    
    I'm not yet sure why the testsuite didn't abort earlier, noticing that
    there was no opt compiler. One possibility is that using "test -e"
    without argument (which happens if variables are empty and not quoted
    inside shell scripts) succeeds. This will have to be checked.
    
    v2: don't remove the bytecode entry (bad copy-paste)
    build: make CAMLOPT_BIN default to "no-opt-compiler-available".
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14278 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02