Fix BULLET_INCLUDE_DIRS in DARTCoreConfig.cmake

The fix from #352 doesn't work if the downstream code
doesn't have the HAVE_BULLET_COLLISION variable defined.
This change just inserts the BULLET_INCLUDE_DIRS
variable into DARTCore_INCLUDE_DIRS
and trusts that it will be blank and not affect
anything if it is unset.

Closes #351.

cmake/DARTCoreConfig.cmake.in

@@ -11,12 +11,9 @@ get_filename_component(CURRENT_DIR "${CMAKE_CURRENT_LIST_FILE}" PATH)
 set(DARTCore_INCLUDE_DIRS
   "@CMAKE_INSTALL_PREFIX@/include"
   "@Eigen_INCLUDE_DIRS@"
+  @BULLET_INCLUDE_DIRS@
 )
 
-if(HAVE_BULLET_COLLISION)
-  set(DARTCore_INCLUDE_DIRS ${DARTCore_INCLUDE_DIRS} "@BULLET_INCLUDE_DIRS@")
-endif()
-
 set(DARTCore_LIBRARY_DIRS "@CMAKE_INSTALL_PREFIX@/lib" "@Boost_LIBRARY_DIRS@")
 
 include("${CURRENT_DIR}/DARTCoreTargets.cmake")