Custom lib/header path doesn't work as intended in case pkg-config is absent

[ShinyMacadamia]

There's a bunch of variables being set here:

        set(${_component}_DEFINITIONS ${PC_${_component}_CFLAGS_OTHER} CACHE STRING "The ${_component} CFLAGS.")
        set(${_component}_VERSION ${PC_${_component}_VERSION} CACHE STRING "The ${_component} version number.")
        set(${_component}_LIBRARY_DIRS ${PC_${_component}_LIBRARY_DIRS} CACHE STRING "The ${_component} library dirs.")
        set(${_component}_LIBRARIES ${PC_${_component}_LIBRARIES} CACHE STRING "The ${_component} libraries.")

The problem these variables are set only via pkg-config:

        set(${_component}_LIBRARY_DIRS ${PC_${_component}_LIBRARY_DIRS} CACHE STRING "The ${_component} library dirs.")
        set(${_component}_LIBRARIES ${PC_${_component}_LIBRARIES} CACHE STRING "The ${_component} libraries.")

Thus, it's impossible to set the custom PC_FFMPEG_INCLUDE_DIRS/ PC_FFMPEG_LIBRARY_DIRS . The latter does basically nothing.
I'm not even sure what's the point to make those variables custom since they have no effect and pkg-config should find headers/.so regardless. Correct me if i'm wrong but i was not able to find a way to set a custom path to library/headers without pkg-config.