Log Message:
multimedia/ffmpeg4: fix USE_LANGUAGES

multimedia/ffmpeg4 does not need -std=gnu99, only ffplay4 does.

Since mk/compiler.mk is protected by a multiple-inclusion guard, any
later assignment to USE_LANGUAGES has no effect except being confusing.
The additional languages show up in "bmake show-var" but at the point of
evaluation, they had not been set yet.

This can be detected by several pkgsrc tools:

pkglint says:

WARN: ../../multimedia/ffmpeg4/Makefile.common:14: Modifying
  USE_LANGUAGES after including ../../mk/compiler.mk has no effect.

Adding a manual check to mk/compiler.mk results in:

bmake[1]: "mk/compiler.mk" line 273: warning:
  For multimedia/ffmpeg4, only languages "c" are used,
  the others in "c99" were defined too late.

Making the variable USE_LANGUAGES read-only after including
mk/compiler.mk results in:

bmake[1]: "multimedia/ffmpeg4/Makefile.common" line 14:
  Read-only USE_LANGUAGES cannot be set

(The latter two checks are not yet in the official pkgsrc tree.)