ASAN makes us forget to add -ldl on the LIBS

With ASAN enabled, linking fails on Fedora 22 / gcc 5.1.0, with a missing symbol, since -ldl is lost. James Peach found some references indicating that ASAN does magic with dlopen(), causing this. The suggestion is to change the configure.ac test to test on dlsym() instead, which does not get hijacked by ASAN.