In CrOS it is more convenient to have shared libraries (instead of static
ones). This CL changes bsdiff.gyp file create shared libraries. It also adds
BSDIFF_EXPORT to export symbols visible for public API.

Bug:none
TEST=sudo emerge bsdiff; cros_workon --board=amd64-generic --test bsdiff

Change-Id: I91cc47d860c9221a13b28ad5336bd0bae3495ff9