configure / meson: Move check for linux/btrfs.h to meson.build

This check can be done in a much shorter way in meson.build. And while
we're at it, rename the #define to HAVE_BTRFS_H to match the other
HAVE_someheader_H symbols that we already have.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201118171052.308191-7-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>