Fix pthread stack size on 32-bit platforms

For some reason which remains mysterious to me the shared library
which calls pthread_create() must be linked with -pthread.  If this
is not done on 32-bit system the default ulimit stack size is used.
Surprisingly, on a 64-bit system the stack limit specified by the
pthread_attr is honored even when -pthread is not passed when linking
the shared library.

lib/libzpool/Makefile.am

@@ -153,3 +153,5 @@ libzpool_la_SOURCES = \
 	${top_srcdir}/module/zfs/include/sys/fm/protocol.h \
 	${top_srcdir}/module/zfs/include/sys/fm/util.h \
 	${top_srcdir}/module/zfs/include/sys/fm/fs/zfs.h
+
+libzpool_la_LDFLAGS = -pthread