latest cmake(since v3.12) fail to find bullet installed to custom path
as title, FindBullet.cmake doesn't take environment variables, so it doesn't know custom bullet path without changing cmake file.
just add a few lines in FindBullet.cmake will solve this issue, say:
if (NOT BULLET_ROOT)
set(ENV_BULLET_ROOT $ENV{BULLET_ROOT})
if (ENV_BULLET_ROOT)
set(BULLET_ROOT ${ENV_BULLET_ROOT})
endif (ENV_BULLET_ROOT)
endif ()
before
macro(_FIND_BULLET_LIBRARY _var)