Fix compilations options problem when building
for mingw32.
 * configure.ac: Removed some LDFLAGS preventing test for
   librestbed to work. Also added -lws2_32 as restbed needs
   it.
 * src/Makefile.am: LDFLAGS removed from configure.ac are
   included here.

Change-Id: Ie6256663778eb1376d4ae7930cbc94e7a240c714