Make the namespace cleanliness check work on Mac OS X.

Fix an old problem: the check_objs target would fail on Mac OS X because
exported symbols on that platform are prefixed with "_".

Mmmake.common.in:
    Allow object symbols to have an optional system-specific prefix.

    Delete a workaround for a problem that was fixed five years ago.

configure.ac:
    On Darwin systems set the appropriate prefix.
Convert .cvsignore -> .gitignore files in tests directory.

tests/*/.cvsignore:
    As above.

library/.gitignore:
    Ignore files generated by the namespace cleanliness check.
Fix bug #261: document how to build the compiler in non-C grades.

README.CSharp:
README.Java:
README.Erlang:
	Document how to build the Mercury compiler using the non-C grades.
Fix typos in documentation for building the compiler in non-C grades.

README.CSharp:
README.Erlang:
README.Java:
	As above.
Fix bug #226: the library predicate lazy.read_if_val/2 was incorrectly declared
as pure.  The reason for it being impure rather than semipure is to avoid any
goals containing calls to force/1 being reordered with respect to it.

library/lazy.m:
    Make read_if_val/2 as impure.

deep_profiler/autopar_types.m:
     Confrom to the above change.
Merge branch 'master' of https://github.com/Mercury-Language/mercury