Fix executable stripping on Mac OS X.

Stripping executables on Mac OS X that were built with --mercury-linkage=shared
was resulting in the symbols for the global variables introduced for runtime
trace goal env flags being incorrectly removed.  We need to pass the "-x" flag
to strip to prevent this happening.  This diff adds the ability to pass
different flags to strip depending on the value of --mercury-linkage.

compiler/options.m:
	Add two new options, --strip-executable-{shared,static}-flags, that
	specify what flags are passed to the strip command when --mercury-linkage
	is set to shared or static respectively.

	Add a new option to test that the compiler is sufficiently up-to-date.

compiler/compile_target_code.m:
	Implement the new options.

doc/user_guide.texi:
	Document the new options.

NEWS:
	Adjust the entry for the --strip-executable-command option to mention
	the additional two new options.