Top level build scripts do not build an external library if it is already compiled and installed. If you need to rebuild one of those libraries you have to use --rebuild-LIBRARY option.

This behaviour aims to decrease the amount of time required to re-build the FFmpegKit library.

PS: Note that adding --rebuild-LIBRARY option alone does not enable a library, only --enable-LIBRARY and --full options can do it. Additionally FFmpeg and FFmpegKit libraries are mandatory and always built, they do not support --reconf option.