CMake does not TARGET_LINK_LIBRARIES if BUILD_SHARED_LIBS=OFF

[glebm]

TARGET_LINK_LIBRARIES should also be present for static linking

Example:

IF (BUILD_SHARED_LIBS)
  TARGET_LINK_LIBRARIES(BulletCollision LinearMath)
ENDIF (BUILD_SHARED_LIBS)

This should just be:

TARGET_LINK_LIBRARIES(BulletCollision LinearMath)

Otherwise, it's difficult to use Bullet statically from another project, the user must manually specify the correct order and all the dependencies. This also means when installing static .a libraries, the generated pkg-config files are wrong.