Fix bullet

[wmisha]

This is required to get this building with CMake 3.1. CMake 3.1 changes the way that link libraries are specified; a single target can only have one link library, not a list of them, and if a list is required then synthetic targets are needed to represent them on a per-entry basis.

[codemercenary]

Merge, or close?

[yeswalrus]

@jhe- Could you take a look at this change? I verified it working on my machine...

[wmisha]

I get this error now when configuring a dependent app (VRIntro). It worked with my commit.

-- Could NOT find Bullet (missing: BULLET_DYNAMICS_LIBRARY BULLET_COLLISION_LIB
RARY BULLET_MATH_LIBRARY BULLET_SOFTBODY_LIBRARY)
-- Could NOT find BULLET_COLLISION (missing: BULLET_COLLISION_LIBRARY BULLET_CO
LLISION_LIBRARY_DEBUG)
-- Could NOT find BULLET_DYNAMICS (missing: BULLET_DYNAMICS_LIBRARY BULLET_DYNA
MICS_LIBRARY_DEBUG)
-- Could NOT find BULLET_MATH (missing: BULLET_MATH_LIBRARY BULLET_MATH_LIBRARY
_DEBUG)
-- Configuring done
CMake Error at source/VRIntro/CMakeLists.txt:52 (add_executable):
Target "VRIntro" links to target "Bullet::Collision" but the target was not
found. Perhaps a find_package() call is missing for an IMPORTED target, or
an ALIAS target is missing?

CMake Error at source/VRIntro/CMakeLists.txt:52 (add_executable):
Target "VRIntro" links to target "Bullet::Dynamics" but the target was not
found. Perhaps a find_package() call is missing for an IMPORTED target, or
an ALIAS target is missing?

CMake Error at source/VRIntro/CMakeLists.txt:52 (add_executable):
Target "VRIntro" links to target "Bullet::Math" but the target was not
found. Perhaps a find_package() call is missing for an IMPORTED target, or
an ALIAS target is missing?