Mention DragonFly in README.md, and fix build guide for {Free/DragonFly}BSD.
Mention DragonFly in README.md, and fix build guide for {Free/DragonFly}BSD.

The Makefile requires GNU make to build, so users will have to run gmake
to compile PCM. If 'gmake' is not available on users' systems, they'll
usually have to install it from ports, e.g. by running 'pkg install gmake'.

opcm/pcm,opcm/pcm,opcm/pcm,opcm/pcm