Fix bullet linking using pkg-config and BULLET_CFLAGS

This adds a step to search for bullet using pkg-config on
UNIX systems. This is necessary to identify the CFLAGS
which are not provided by the FindBullet module.
The CFLAGS are needed to distinguish between single and
double precision sizes of btScalar.
Merge pull request #176 from scpeters/bullet_pkg_config

Fix bullet linking using pkg-config and BULLET_CFLAGS