cmake_minimum_required(VERSION 3.0)

set(ffmpeg_prefix "${CMAKE_BINARY_DIR}/foobar")

file(WRITE ${ffmpeg_prefix}/lib/libavcodec.so)

add_library(ext_ffmpeg_libavcodec SHARED IMPORTED)
set_target_properties(ext_ffmpeg_libavcodec PROPERTIES
    IMPORTED_LOCATION "${ffmpeg_prefix}/lib/libavcodec.so")

file(WRITE foo.cpp "int main() {}")
add_executable(foo foo.cpp)
target_link_libraries(foo PRIVATE ext_ffmpeg_libavcodec)