How to specify package location in cmake file?

Question

I have two versions of the same library (g2o) on my Ubuntu 18.04 pc. One is built with catkin_make in a ROS workspace (/home/riccardo/catkin_ws/src/g2o). The other one is in the directory of the project I'm trying to build (/home/myproject/g2o). As I said, I'm trying to build myproject with catkin_build, and this second version of the g2o library. However, I see that catkin_build finds the first version of the g2o library, the one in the ROS workspace.

I have a CMakeLists.txt under the myproject directory and I tried different ways to specify the exact package location:

• First, I tried with:

find_package(G2O REQUIRED PATHS ${PROJECT_SOURCE_DIR}/g2o)

In this case the error is the following:

CMake Error at /home/myproject/CMakeLists.txt:28 (find_package):
  Could not find a package configuration file provided by "G2O" with any of
  the following names:

    G2OConfig.cmake
    g2o-config.cmake

  Add the installation prefix of "G2O" to CMAKE_PREFIX_PATH or set "G2O_DIR"
  to a directory containing one of the above files.  If "G2O" provides a
  separate development package or SDK, be sure it has been installed.

• Then, I also tried with:

LIST(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/g2o/cmake_modules)
SET(G2O_ROOT ${PROJECT_SOURCE_DIR}/g2o)
find_package(G2O REQUIRED)

And in this case the other g2o version was used.

Can you help me please?

EDIT 1

I think there is a related question on Stackoverflow: How to clone, build, and link g2o framework in C++ However I do not have the directory specified by ${CMAKE_PREFIX_INTALL} in /home/user/ . I do not understand what I have to do...

EDIT 2

For building and installing the g2o library I launched the following commands in the g2o root directory:

• mkdir build
• cd build
• cmake ../
• make