cmake linker error with library installed in custom path - cmake

Question

I have compiled and installed with CMake the library SDL_bgi to a custom prefix /custom/prefix/. This library uses SDL2.
Now I want to use it in another project with the structure below but I get a linker error when I compile with make:
/usr/bin/c++ CMakeFiles/test.dir/test.cpp.o -o test -Wl,-rpath,/custom/prefix/lib: /custom/prefix/lib/libSDL_bgi.so
/usr/bin/ld: /custom/prefix/lib/libSDL_bgi.so: undefined reference to `SDL_DestroyWindow'
/usr/bin/ld: /custom/prefix/lib/libSDL_bgi.so: undefined reference to `SDL_CreateRenderer'
I have also written the file cmake/modules/FindSDL_bgi.cmake so that may be wrong as well.
If I compile with the following command I can compile correctly:
g++ test.cpp -I . -lSDL_bgi -lSDL2 -I /custom/prefix/include/ -L /custom/prefix/lib/
What am I doing wrongly?
Project structure:
cmake/modules/FindSDL_bgi.cmake
src/test/CMakeLists.txt
src/test/test.cpp
CMakeLists.txt
Libraries:
/usr/lib/libSDL.so
/usr/include/SDL.h
/custom/prefix/lib/libSDL_bgi.so
/custom/prefix/include/graphics.h
/custom/prefix/include/SDL2/libSDL_bgi.h
cmake/modules/FindSDL_bgi.cmake:
# - Try to find LibXml2
# Once done this will define
# SDL_BGI_FOUND - System has LibXml2
# SDL_BGI_INCLUDE_DIRS - The LibXml2 include directories
# SDL_BGI_LIBRARIES - The libraries needed to use LibXml2
# Hardcoded for now
set(SDL_BGI_PATH
/custom/prefix/
)
set(SDL_BGI_SEARCH_PATHS
/usr
/usr/local
/opt
${SDL_BGI_PATH}
)
find_path(SDL_BGI_INCLUDE_DIR graphics.h
HINTS
$ENV{SDL2DIR}
PATH_SUFFIXES include
PATHS ${SDL2_SEARCH_PATHS}
)
find_library(SDL_BGI_LIBRARY
NAMES SDL_bgi
HINTS
$ENV{SDL2DIR}
PATH_SUFFIXES lib64 lib
PATHS ${SDL2_SEARCH_PATHS}
)
include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(SDL_bgi REQUIRED_VARS SDL_BGI_LIBRARY SDL_BGI_INCLUDE_DIR)
CMakeLists.txt
cmake_minimum_required(VERSION 3.15)
project(programmi_kennedy)
list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake/modules")
set(COMPAT_HEADERS
${CMAKE_CURRENT_SOURCE_DIR}/include/
)
find_package(SDL_bgi REQUIRED)
add_subdirectory(src/test)
src/CMakeLists.txt:
add_executable(test test.cpp)
target_include_directories(test PUBLIC ${SDL_BGI_INCLUDE_DIR})
target_link_libraries(test PRIVATE ${SDL_BGI_LIBRARY})
install(TARGETS test DESTINATION bin)
/custom/prefix/include/graphics.h:
#include <SDL2/SDL_bgi.h>

Answer 1

What I was missing is to link to SDL2 with find_package(SDL2 REQUIRED) and link to SDL2::SDL2. (I did try to link to ${SDL2_LIBRARIES} but the syntax is different now). Thanks to #KamilCuk to point me to the right direction.
EDIT:
I changed the FindBGI_sdl.cmake module in order to search for the dependencies (SDL2) and link against them using the INTERFACE keyword. In this way the target test can link only against SDL_bgi and have the dependencies resolved automatically.
src/CMakeLists.txt:
add_executable(test test.cpp)
target_link_libraries(test PRIVATE SDL_bgi::SDL_bgi)
install(TARGETS test DESTINATION bin)
cmake/modules/FindSDL_bgi.cmake:
# Distributed under the OSI-approved BSD 3-Clause License. See accompanying
# file Copyright.txt or https://cmake.org/licensing for details.
#[=======================================================================[.rst:
FindSDL_bgi
-------
Finds the SDL_bgi library.
Imported Targets
^^^^^^^^^^^^^^^^
This module provides the following imported targets, if found:
``SDL_bgi::SDL_bgi``
The SDL_bgi library
Result Variables
^^^^^^^^^^^^^^^^
This will define the following variables:
``SDL_bgi_FOUND``
True if the system has the SDL_bgi library.
``SDL_bgi_VERSION``
The version of the SDL_bgi library which was found.
``SDL_bgi_INCLUDE_DIRS``
Include directories needed to use SDL_bgi.
``SDL_bgi_LIBRARIES``
Libraries needed to link to SDL_bgi.
Cache Variables
^^^^^^^^^^^^^^^
The following cache variables may also be set:
``SDL_bgi_INCLUDE_DIR``
The directory containing ``foo.h``.
``SDL_bgi_LIBRARY``
The path to the SDL_bgi library.
#]=======================================================================]
find_package(SDL2 REQUIRED)
find_package(PkgConfig)
pkg_check_modules(PC_SDL_bgi QUIET SDL_bgi)
find_path(SDL_bgi_INCLUDE_DIR
NAMES graphics.h
PATHS ${PC_SDL_bgi_INCLUDE_DIRS}
)
find_library(SDL_bgi_LIBRARY
NAMES SDL_bgi
PATHS ${PC_SDL_bgi_LIBRARY_DIRS}
)
set(SDL_bgi_VERSION ${PC_SDL_bgi_VERSION})
include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(SDL_bgi
FOUND_VAR SDL_bgi_FOUND
REQUIRED_VARS
SDL_bgi_LIBRARY
SDL_bgi_INCLUDE_DIR
VERSION_VAR SDL_bgi_VERSION
)
if(SDL_bgi_FOUND AND NOT TARGET SDL_bgi::SDL_bgi)
add_library(SDL_bgi::SDL_bgi UNKNOWN IMPORTED)
set_target_properties(SDL_bgi::SDL_bgi PROPERTIES
IMPORTED_LOCATION "${SDL_bgi_LIBRARY}"
INTERFACE_COMPILE_OPTIONS "${PC_SDL_bgi_CFLAGS_OTHER}"
INTERFACE_INCLUDE_DIRECTORIES "${SDL_bgi_INCLUDE_DIR}"
)
target_link_libraries(SDL_bgi::SDL_bgi INTERFACE SDL2::SDL2)
endif()
mark_as_advanced(
SDL_bgi_INCLUDE_DIR
SDL_bgi_LIBRARY
SDL2_DIR
)
Useful references:
https://pabloariasal.github.io/2018/02/19/its-time-to-do-cmake-right/
https://cmake.org/cmake/help/v3.17/manual/cmake-developer.7.html