Update CMakeList to include bullet library

Only by adding the bullet library I was able to complete compilation on Linux (Linux Mint 17.2). This solves issue 1801:
cocos2d#1801

CMakeLists.txt

@@ -33,6 +33,7 @@ set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_CURRENT_SOURCE_DIR}/framewor
 include(CocosBuildHelpers)
 
 option(USE_CHIPMUNK "Use chipmunk for physics library" ON)
+option(USE_BULLET "Use bullet for physics3d library" ON)
 option(USE_BOX2D "Use box2d for physics library" OFF)
 option(DEBUG_MODE "Debug or release?" ON)
 option(BUILD_EXTENSIONS "Build extension library" ON)
@@ -193,6 +194,24 @@ else()
   add_definitions(-DCC_ENABLE_BOX2D_INTEGRATION=0)
 endif(USE_BOX2D)
 
+# Bullet (not prebuilded, exists as source)
+if(USE_BULLET)
+  if(USE_PREBUILT_LIBS)
+    add_subdirectory(frameworks/cocos2d-x/external/bullet)
+    set(BULLET_INCLUDE_DIRS ${CMAKE_CURRENT_SOURCE_DIR}/frameworks/cocos2d-x/external/bullet)
+    set(BULLET_LIBRARIES bullet)
+  else()
+    cocos_find_package(bullet BULLET REQUIRED)
+    set(BULLET_LIBRARIES bullet)
+  endif()
+  message(STATUS "Bullet include dirs: ${BULLET_INCLUDE_DIRS}")
+  add_definitions(-DCC_ENABLE_BULLET_INTEGRATION=1)
+  add_definitions(-DCC_USE_PHYSICS=1)
+else()
+  add_definitions(-DCC_ENABLE_BULLET_INTEGRATION=0)
+  add_definitions(-DCC_USE_3D_PHYSICS=0)
+endif(USE_BULLET)
+
 # Tinyxml2 (not prebuilded, exists as source)
 if(USE_PREBUILT_LIBS)
   add_subdirectory(frameworks/js-bindings/cocos2d-x/external/tinyxml2)