megorit

if (${CMAKE_SIZEOF_VOID_P} STREQUAL 4)
    message("It's 32 bit build")
elseif (${CMAKE_SIZEOF_VOID_P} STREQUAL 8)
    message("It's 64 bit build")
endif ()

set(CMAKE_LIBRARY_PATH C:/Users/march/Desktop/)

find_library(av_codec
    NAMES
        avcodec
    PATH_SUFFIXES
        ffmpeg-4.3-win64-dev/lib/
    REQUIRED
)

message(${av_codec})

• добавил каталог собранной библиотеки в каталог проекта
  
• открыл CMakeLists.txt в своём проекте и дописал:
  

  target_include_directories(${PROJECT_NAME} PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/path_to_include_dir)
  target_link_directories(${PROJECT_NAME} PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/path_to_link_dir)
  target_link_libraries(${PROJECT_NAME} PUBLIC libname)