[CMake] find_package(), sources only

[Daniel Dekkers]

Hi Hendrik,

Could you write a few more lines. I want to understand,... but I don't. ;-)

We only have one copy of Bullet "as a bundle" present on the system. Let's
say you just downloaded Bullet.
With BULLET_ROOT, we set the root path to that copy and do a
find_package(BULLET), it returns with:
   BULLET_FOUND (only true if everything is found, no, because the libs
aren't built)
   BULLET_INCLUDE_DIR (set if the sources are found, ok, we downloaded the
source distribution)
   BULLET_COLLISION_LIBRARY[_DEBUG] (not defined, we haven't built Bullet
(yet))
   BULLET_DYNAMICS_LIBRARY[_DEBUG] (not defined, we haven't built Bullet
(yet))
   BULLET_MATH_LIBRARY[_DEBUG] (not defined, we haven't built Bullet (yet))