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 ignores
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
Add patch to write use dbl prec flag to btScalar.h

If the CMake option USE_DOUBLE_PRECISION is true, btScalar.h will be configured
during build to contain the lines
```
  #ifndef BT_USE_DOUBLE_PRECISION
  #define BT_USE_DOUBLE_PRECISION
  #endif
```
Otherwise, btScalar.h will not contain those lines. This lets us sidestep the
issue of the pods macros not reading the preprocessor flags from pkg-config
files, and still not have to define the preprocessor flag everywhere that
includes the bullet headers.
Make patching `btScalar.h` more robust.

This also makes `bullet_double_precision_patch` legible.
Merge pull request #2 from avalenzu/double-precision

Add patch to write use dbl prec flag to btScalar.h
Update GJK accuracy patch