Delete FindBullet.cmake; we use cmake's version which has been available since 3.0.2 and we have a requirement of 3.1.0

https://cmake.org/cmake/help/v3.0/module/FindBullet.html

cmake/FindBullet.cmake

@@ -1,73 +0,0 @@
-# - Try to find the Bullet physics engine
-#
-# This module accepts the following env variables
-#  BULLET_ROOT - Can be set to bullet install path or Windows build path
-#
-# Once done this will define
-#  Bullet_FOUND         - System has the all required components.
-#  Bullet_INCLUDE_DIRS  - Include directory necessary for using the required components headers.
-#  Bullet_LIBRARIES     - Link these to use the required bullet components.
-#  Bullet_VERSION       - Version of libbullet
-#
-# For each of the components
-#   - LinearMath
-#   - BulletCollision
-#   - BulletSoftBody
-#   - BulletDynamics
-#
-# Copyright (c) 2009, Philip Lowman <philip at yhbt.com>
-# Modified for OpenMW to parse BT_BULLET_VERSION.
-#
-# Redistribution AND use is allowed according to the terms of the New
-# BSD license.
-# For details see the accompanying COPYING-CMAKE-SCRIPTS file.
-
-include(LibFindMacros)
-
-# Macro: _internal_find_bullet_library
-# Checks for the given component by invoking pkgconfig etc.
-macro(_internal_find_bullet_library _lib)
-    libfind_pkg_detect(Bullet_${_lib} bullet
-        FIND_LIBRARY ${_lib}
-            HINTS $ENV{BULLET_ROOT}
-            PATH_SUFFIXES lib
-    )
-    libfind_process(Bullet_${_lib})
-endmacro()
-
-set(_known_components LinearMath BulletCollision BulletSoftBody BulletDynamics)
-
-# Check if the required components were found and add their stuff to the Bullet_* vars.
-foreach (_component ${Bullet_FIND_COMPONENTS})
-    list(FIND _known_components ${_component} _known_component)
-    if (_known_component EQUAL -1)
-        message(FATAL_ERROR "Unknown component '${_component}'")
-    endif()
-
-    set(Bullet_${_component}_Debug_FIND_QUIETLY TRUE) # don't spam messages with optional Debug component
-    _internal_find_bullet_library(${_component})
-    _internal_find_bullet_library(${_component}_Debug)
-
-    if (Bullet_${_component}_Debug_FOUND)
-        set(Bullet_LIBRARIES ${Bullet_LIBRARIES} optimized ${Bullet_${_component}_LIBRARIES} debug ${Bullet_${_component}_Debug_LIBRARIES})
-    else()
-        set(Bullet_LIBRARIES ${Bullet_LIBRARIES} ${Bullet_${_component}_LIBRARIES})
-    endif()
-endforeach()
-
-libfind_pkg_detect(Bullet bullet
-    FIND_PATH btBulletCollisionCommon.h
-        HINTS $ENV{BULLET_ROOT}
-        PATH_SUFFIXES include/bullet
-)
-set(Bullet_INCLUDE_DIRS ${Bullet_INCLUDE_DIR})
-libfind_version_header(Bullet LinearMath/btScalar.h BT_BULLET_VERSION)
-
-FIND_PACKAGE_HANDLE_STANDARD_ARGS(Bullet
-    FOUND_VAR Bullet_FOUND
-    VERSION_VAR Bullet_VERSION
-    HANDLE_COMPONENTS
-    REQUIRED_VARS
-        Bullet_LIBRARIES
-        Bullet_INCLUDE_DIR
-)