Fix custom ffmpeg library/header building

[ShinyMacadamia]

This PR aims to resolve this issue:
#157
In order to resolve it, i conditionally check whether PC_FFMPEG_LIBRARY_DIRS and PC_FFMPEG_INCLUDE_DIRS are present. If library is found, it sets the variables manually:

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

I'm almost certain this also should be checked and set manually in case PC_FFMPEG_INCLUDE_DIRS is present:

      find_path(${_component}_INCLUDE_DIRS ${_header}
            HINTS
            ${PC_${_component}_INCLUDEDIR}
            ${PC_${_component}_INCLUDE_DIRS}
            ${PC_FFMPEG_INCLUDE_DIRS}
            PATH_SUFFIXES
            ffmpeg
    )

like this:

    find_path(${_component}_INCLUDE_DIRS ${_header}
         PATHS
         ${PC_FFMPEG_INCLUDE_DIRS}
         NO_DEFAULT_PATH
         REQUIRED # perhaps?
)

About formatting. I didn't find any cmake-format configs in the repo. Because of this, cmake-formatter with my own settings was applied. I'm not even sure how to format this exact cmake file.
Any suggestions are appreciated!