Strip executable as a separate step on Mac OS X.

We had stopped passing the linker strip flag on Mac OS X because it had
become deprecated and useless there, but we can still strip the binary
with the `strip' tool.

This commit changes compile_target_code.m to do so, but NOT the
`ml' shell script.

compiler/options.m:
doc/user_guide.texi:
	Add option --strip-executable-command.

configure.ac:
scripts/Mercury.config.in:
	Set --strip-executable-command="strip" on Darwin platforms,
	or an empty string elsewhere.

compiler/compile_target_code.m:
	Invoke the --strip-executable-command separately after linking
	an executable if necessary.

scripts/ml.in:
	Fix a bug: use configured value of LD_STRIP_FLAG instead of
	assuming "-s".