On Ubuntu 11.10 "Oneiric" with gcc 4.6.1, compiling the btrfs tools from git
fails for me with the following error:

[lenz@metis btrfs-progs]% make
bash version.sh
gcc -lpthread -g -Werror -Os -o btrfs btrfs.o btrfs_cmds.o scrub.o \
		ctree.o disk-io.o radix-tree.o extent-tree.o print-tree.o root-tree.o
dir-item.o file-item.o inode-item.o inode-map.o crc32c.o rbtree.o
extent-cache.o extent_io.o volumes.o utils.o btrfs-list.o btrfslabel.o  -luuid
scrub.o: In function `scrub_start':
/space/tmp/btrfs-progs/scrub.c:1342: undefined reference to `pthread_create'
/space/tmp/btrfs-progs/scrub.c:1360: undefined reference to `pthread_create'
/space/tmp/btrfs-progs/scrub.c:1374: undefined reference to `pthread_join'
/space/tmp/btrfs-progs/scrub.c:1430: undefined reference to `pthread_cancel'
/space/tmp/btrfs-progs/scrub.c:1432: undefined reference to `pthread_join'
collect2: ld returned 1 exit status
make: *** [btrfs] Error 1

The following change fixed this for me, not sure if it's the appropriate approach:

diff --git a/Makefile b/Makefile
index edee1a0..9e3a06f 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 CC = gcc
 AM_CFLAGS = -Wall -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2
-CFLAGS = -g -Werror -Os
+CFLAGS = -g -Werror -Os -pthread
 objects = ctree.o disk-io.o radix-tree.o extent-tree.o print-tree.o \
          root-tree.o dir-item.o file-item.o inode-item.o \
          inode-map.o crc32c.o rbtree.o extent-cache.o extent_io.o \


-- 
  Lenz Grimmer <lenz@grimmer.com> - http://www.lenzg.net/