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>
diff --git a/meson.build b/meson.build
index 04cdea7..fba6413 100644
--- a/meson.build
+++ b/meson.build
@@ -836,6 +836,7 @@
 config_host_data.set('QEMU_VERSION_MINOR', meson.project_version().split('.')[1])
 config_host_data.set('QEMU_VERSION_MICRO', meson.project_version().split('.')[2])
 
+config_host_data.set('HAVE_BTRFS_H', cc.has_header('linux/btrfs.h'))
 config_host_data.set('HAVE_DRM_H', cc.has_header('libdrm/drm.h'))
 config_host_data.set('HAVE_PTY_H', cc.has_header('pty.h'))
 config_host_data.set('HAVE_SYS_IOCCOM_H', cc.has_header('sys/ioccom.h'))