fix compilation on ubuntu

See
https://stackoverflow.com/questions/14329967/missing-a-library-in-ldd-after-using-gcc-l
https://savannah.gnu.org/forum/forum.php?forum_id=5655

make

 #!/bin/sh
 
-test green.pyx -ot green.so || ( cython --cplus green.pyx && g++ -shared -pthread -fPIC -fwrapv -O3 -Wall -lgsl -lgslcblas -fno-strict-aliasing -I/usr/include/python2.7 -o green.so green.cpp )
+test green.pyx -ot green.so || ( cython --cplus green.pyx && g++ -shared -pthread -fPIC -fwrapv -O3 -Wall -Wl,--no-as-needed -lgsl -lgslcblas -fno-strict-aliasing -I/usr/include/python2.7 -o green.so green.cpp )
 python test.py