audio/pianobar Makefile cleanup, minor tweaks

Add localbase and remove unneeded requisites and variables
Upstreamed changed backend to ffmpeg (avcodec) back in 2014.
Commit: c872f00508ce4afe3b8ec863b23595c31fd8b4be [1]

Note that -std=c99 is set by pianobar's original Makefile already,
hence it was redundant. [1, see reviews.f.o discussion, URL below]

While here, add V=1 to MAKE_ENV to see the build commands in logs,
and run STRIP_CMD on the installed binary. [mandree@]

Submitted by: Daniel Engberg [1]
Approved by: jhixson@FreeBSD.org (maintainer timeout, 45 days)
Differential Revision: https://reviews.freebsd.org/D22269