Make sure that libdm usage always brings in pthread libraries, both in
pkgconfig and during manual build.

--- tools/Makefile.in
+++ tools/Makefile.in
@@ -87,6 +87,7 @@
   INSTALL_LVM_TARGETS += install_tools_static
   INSTALL_DMSETUP_TARGETS += install_dmsetup_static
   INSTALL_CMDLIB_TARGETS += install_cmdlib_static
+  STATIC_LIBS += @PTHREAD_LIBS@
 endif
 
 LVMLIBS = $(LVMINTERNAL_LIBS)
@@ -118,6 +119,10 @@
 
 include $(top_builddir)/make.tmpl
 
+ifeq ("@STATIC_LINK@", "yes")
+  STATIC_LIBS += @PTHREAD_LIBS@
+endif
+
 device-mapper: $(TARGETS_DM)
 
 CFLAGS_dmsetup.o += $(UDEV_CFLAGS) $(EXTRA_EXEC_CFLAGS)