Estimated hours taken: 0.5
Branches: main, release

Various changes to make it easier to build the extras.

extras/dynamic_linking/Mmakefile:
	Don't require the existence of "../../../mercury/browser".

extras/posix/README:
	Mention that it may be necessary to set MLLIBS.

extras/references/Mmakefile:
	Don't include the "asm_fast.tr" grade in LIBGRADES, since by
	default the Mercury standard library doesn't get installed in
	that grade.