pthread flag missing in compilation of list-backends

[jafet-chaves]

These are the commands to replicate:

./autogen.sh --disable-docs --enable-tflite
make
sudo make install
cd examples/external
make

Which gives:

/usr/local/lib/libr2inference-0.0.so: undefined reference to 'pthread_create'
/usr/local/lib/libr2inference-0.0.so: undefined reference to 'pthread_join'
collect2: error: ld returned 1 exit status
Makefile:25: recipe for target 'list-backends' failed
make: *** [list-backends] Error 1