Fix linking of libiscsi to libpthread

libiscsi utilizes a bunch of POSIX threads functions but does
not link against libpthread as it should nor the pkg-config file
properly listing libpthread as a static link time dependency.

lib/Makefile.am

@@ -21,8 +21,14 @@ if HAVE_LINUX_ISER
 libiscsipriv_la_SOURCES += iser.c
 endif
 
+libiscsipriv_la_LIBADD =
+
+if HAVE_PTHREAD_SPIN_LOCKS
+libiscsipriv_la_LIBADD += -lpthread
+endif
+
 if HAVE_LINUX_ISER
-libiscsipriv_la_LIBADD = -libverbs -lrdmacm -lpthread
+libiscsipriv_la_LIBADD += -libverbs -lrdmacm
 endif
 
 libiscsipriv_la_LDFLAGS = -no-undefined