This makes the macro usage consitent between bullet.pc.in and BulletConfig.cmake.in, previously if the CMAKE_INSTALL_PREFIX was set you would end up with -I/usr/usr/include/bullet rather then -I/usr/include/bullet
Fixes #321
1) sample directions over a unit sphere and 2) add a contact, using mpr add missing bullet.pc.cmake file (for old Bullet 2.x)