libpostproc is licensed under the GNU General Public License
version 2 or later (GPL v2+). See the file COPYING.GPLv2 for details.
Should you, for whatever reason, prefer to use version 3 of the GPL, then
the configure parameter --enable-version3 will activate this licensing option
for you. Read
COPYING.GPLv3 to learn the exact legal terms that apply in this case.
There are a handful of files under other licensing terms, namely:
tests/reference.pnmis under the expat license.
FFmpeg can be combined with a number of external libraries, which sometimes affect the licensing of binaries resulting from the combination.