Fixed install

PLUMED_STATIC_DEPENDENCIES was changed in 2.2. I restore here the value that
it had in 2.0 and 2.1 to keep it compatible with cp2k installation procedure

see https://groups.google.com/d/msgid/plumed-users/CAGZMOouB989WaXVd1E8%3DAVmi_BM7EnSpzN7P-NBjR%2BLXjWYV8A%40mail.gmail.com