For glibc, lua needs an explicit link to libdl as glibc has it separate

Fixes the following error in at least collectd:

ld: usr/lib/liblua.so: undefined reference to `dlopen'
ld: usr/lib/liblua.so: undefined reference to `dlclose'
ld: usr/lib/liblua.so: undefined reference to `dlerror'
ld: usr/lib/liblua.so: undefined reference to `dlsym'

Signed-off-by: Rosen Penev <rosenp@gmail.com>