/usr/lib/x86_64-linux-gnu/pkgconfig/lua51.pc is in liblua5.1-0-dev 5.1.5-8ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/pkgconfig/lua5.1.pc
/usr/lib/x86_64-linux-gnu/pkgconfig/lua5.1.pc