xlib_libpthread-stubs: needs -pthread when linking statically

Fixes e.g. http://autobuild.buildroot.net/results/1eaa694263b74313b0c90c510abcb11d490a3773/

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/x11r7/xlib_libpthread-stubs/xlib_libpthread-stubs.mk

@@ -12,5 +12,9 @@ XLIB_LIBPTHREAD_STUBS_LICENSE_FILES = COPYING
 
 XLIB_LIBPTHREAD_STUBS_INSTALL_STAGING = YES
 
+ifeq ($(BR2_PREFER_STATIC_LIB),y)
+XLIB_LIBPTHREAD_STUBS_CONF_ENV += LDFLAGS="$(TARGET_LDFLAGS) -pthread"
+endif
+
 $(eval $(autotools-package))
 $(eval $(host-autotools-package))