Set -undefined dynamic_lookup when linking on Mac OS X.
Set -undefined dynamic_lookup when linking on Mac OS X.

This fixes #822.

fishman1986/libphonenumber,fishman1986/libphonenumber,keghani/libphonenumber,fishman1986/libphonenumber,google/libphonenumber,keghani/libphonenumber,google/libphonenumber,google/libphonenumber,fishman1986/libphonenumber,lcorbasson/libphonenumber,google/libphonenumber,lcorbasson/libphonenumber,keghani/libphonenumber,keghani/libphonenumber,lcorbasson/libphonenumber,lcorbasson/libphonenumber,google/libphonenumber,keghani/libphonenumber,fishman1986/libphonenumber,google/libphonenumber,lcorbasson/libphonenumber