multimedia/ffmpeg0: Use OPTIONS helpers, Honour CFLAGS

• Use OPTIONS helpers for as many as conditional blocks as possible. Blocks with FFMPEG_* and other variables not supported by the helper framework are not modified.
• Honour CFLAGS for armv6 (= -> ?=)

While I'm here:

• Sort and group common or related Makefile sections where it made sense to do so and improved readability. Put global things up the top and conditional blocks below.
• Improve whitespace alignment for readability.

Approved by: wg (maintainer)
Differential Revision: https://reviews.freebsd.org/D2981