Makefile.am: Add -lpthread to fix build on newer ld in Raring Ringtail

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 98f532e8000af281fa03b24da9ad2fda81755270)

Conflicts:
	src/Makefile.am