Changes bsdiff to be shared library in CrOS.

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
6 files changed