configure: check for dlsym as well

For some reason, when compiling with gcc-asan and a recent enough gcc
version(seen on 5.3+ so far), linking dlopen works without -ldl, but
dlsym fails with:

undefined reference to symbol 'dlsym@@GLIBC_2.2.5'

So this patchs checks for both dlopen and dlsym to work for determining
if -ldl is needed.
