Drop support for lcc as a C compiler.

Drop support for lcc as a C compiler.  lcc itself does not seem to have been
updated since around 2010 and Mercury's support for it has not been updated for
quite a while before that.

Derivatives of lcc, for example lcc-win32, would almost certainly need to be
treated separately anyway, so this change doesn't affect them (i.e. they are
already not supported).

configure.ac:
m4/mercury.m4:
	Do not recognise lcc as a C compiler.

	Unrelated change: remove residual support for a.out executables on Linux.

scripts/mgnuc_file_opts.sh-subr:
	Do not define the macros __EXTENSIONS__ and _GNU_SOURCE if the mgnuc script
	is invoked with --no-ansi.  This was originally done to support lcc on Linux.
	Doing so on *all* systems as this code actually does is not a good idea.
	(If it becomes necessary to reinstate this behaviour on some system, it
	should be controlled from the configure script not here.)

runtime/mercury_wrapper.c:
	Delete a workaround for lcc.

scripts/mgnuc.in:
scripts/ml.in:
compiler/compile_target_code.m:
compiler/handle_options.m:
compiler/globals.m:
compiler/ml_code_gen.m:
	Delete support for lcc.

README.lcc:
	Delete this file.