Set USE_DOUBLE_PRECISION option to 'on'

This *does* put the necessary definition (`-DBT_USE_DOUBLE_PRECISION`) into the
pkg-config file. However, `pods_use_pkg_config_packages`, does *not* read that
portion of the pkg-config file! See RobotLocomotion/cmake/#3.
Add patch to increase accuracy in GJK.

This patch is applied by the Makefile after the bullet source archive is
decompressed.
Merge branch 'double-precision' of https://github.com/avalenzu/bullet-pod
apply patch if patch is newer than existing cmake