build: Compile with -Wall -Wunused only on linux.

build: Compile with -Wall -Wunused only on linux.

On *BSD the old set of CFLAGS causes warnings of the form

	/usr/pkg/include/vorbis/vorbisfile.h:75:21: warning: 'OV_CALLBACKS_DEFAULT' defined but not used
	/usr/pkg/include/vorbis/vorbisfile.h:82:21: warning: 'OV_CALLBACKS_NOCLOSE' defined but not used
	/usr/pkg/include/vorbis/vorbisfile.h:89:21: warning: 'OV_CALLBACKS_STREAMONLY' defined but not used
	/usr/pkg/include/vorbis/vorbisfile.h:96:21: warning: 'OV_CALLBACKS_STREAMONLY_NOCLOSE' defined but not used

This commit avoids these warnings by enabling -Wall and -Wunused only
on Linux systems. Let's also collect these flags in STRICT_CFLAGS
rather than CFLAGS to avoid warnings when compiling the *.cmdline.c
files.