Header not found gazebo/math/Rand.hh

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While following the following question to detect mouse click events, https://answers.gazebosim.org//questi..., I saw my computer cannot find the header file gazebo/math/Rand.hh

Here is my CMakeLists.txt:

cmake_minimum_required(VERSION 2.8 FATAL_ERROR)

find_package(gazebo REQUIRED)
include_directories(${GAZEBO_INCLUDE_DIRS})
link_directories(${GAZEBO_LIBRARY_DIRS})
list(APPEND CMAKE_CXX_FLAGS "${GAZEBO_CXX_FLAGS}")

add_library(hello_world SHARED hello_world.cc)
target_link_libraries(hello_world ${GAZEBO_LIBRARIES})

I am on Ubuntu 18.04, my ROS version is melodic and I have gazebo-9

I tried to find if I have gazebo/math in my include directory. I am not sure if this is the correct approach, but I ran the following commands

locate gazebo | grep include | grep math

This did not show any gazebo/math directory in the includes folder.

But running this command shows that I have gazebo/gui folder in my include directory

locate gazebo | grep include | grep gui