ltdl not found on Darwin

On my computer (MacOS 10.12.2) libtldl is not in my paths (neither the header, nor the dynamic library). As a result, when I do : ./configure && make I got the following error :

ltsmin.cc:20:10: fatal error: 'ltdl.h' file not found
#include <ltdl.h>

Nonetheless, as it is specified on HACKING, ltdl is not required: if it is available, we can use it, otherwise spot should provide a fresh one.

Moreover, when I only specify the path to the ltdl.h file with the following command ./configure CXXFLAGS='-I /Users/etienne/local/bin/MacPorts-2.3.5/include/'

the make step works well...