Log Message:
multimedia/mplayer: Fix cross-build.

This removes our local patch to pass CFLAGS to compile the the
build-time tool codec-cfg.  These CFLAGS don't work for cross-builds,
but all we really need is to make libavutil/avutil.h.  So instead of
patching the makefile, just pass BUILDLINK_CPPFLAGS through into the
HOST_CC invocation that compiles it, in native builds.

And in cross builds, sleazily pass the include path under the
localbase in the destdir, so that we don't have to pull in ffmpeg5 as
a tool dependency -- libavutil/*.h seems to work here, and may
actually be correct because it pulls in libavutil/avconfig.h and
codec-cfg might want that to describe the target system rather than
the build system.