Change the btrfs_noversion check to be automatic

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

Dockerfile

@@ -138,7 +138,7 @@ RUN useradd --create-home --gid docker unprivilegeduser
 
 VOLUME /var/lib/docker
 WORKDIR /go/src/github.com/docker/docker
-ENV DOCKER_BUILDTAGS apparmor selinux btrfs_noversion
+ENV DOCKER_BUILDTAGS apparmor selinux
 
 # Let us use a .bashrc file
 RUN ln -sfv $PWD/.bashrc ~/.bashrc

hack/make.sh

@@ -98,6 +98,14 @@ if [ "$DOCKER_EXECDRIVER" = 'lxc' ]; then
 	DOCKER_BUILDTAGS+=' test_no_exec'
 fi
 
+# test whether "btrfs/version.h" exists and apply btrfs_noversion appropriately
+if \
+	command -v gcc &> /dev/null \
+	&& ! gcc -E - &> /dev/null <<<'#include <btrfs/version.h>' \
+; then
+	DOCKER_BUILDTAGS+=' btrfs_noversion'
+fi
+
 # Use these flags when compiling the tests and final binary
 
 IAMSTATIC='true'

project/PACKAGERS.md

@@ -58,8 +58,7 @@ To build the Docker daemon, you will additionally need:
 * libdevmapper version 1.02.68-cvs (2012-01-26) or later from lvm2 version
   2.02.89 or later
 * btrfs-progs version 3.16.1 or later (unless using an older version is
-  absolutely necessary, in which case 3.8 is the minimum and the note below
-  regarding `btrfs_noversion` applies)
+  absolutely necessary, in which case 3.8 is the minimum)
 
 Be sure to also check out Docker's Dockerfile for the most up-to-date list of
 these build-time dependencies.
@@ -163,12 +162,6 @@ SELinux, you will need to use the `selinux` build tag:
 export DOCKER_BUILDTAGS='selinux'
 ```
 
-If your version of btrfs-progs (also called btrfs-tools) is < 3.16.1, then you
-will need the following tag to not check for btrfs version headers:
-```bash
-export DOCKER_BUILDTAGS='btrfs_noversion'
-```
-
 There are build tags for disabling graphdrivers as well. By default, support
 for all graphdrivers are built in.