Debian [1], FreeBSD [2], and probably other OSs provide pkg-config files for lmdb.
However, brew [3] and upstream [4] don't and don't want to.
Let's fallback to manual detection of LMDB, in case the pkg-config file is not there.
[1] https://salsa.debian.org/debian/lmdb/-/blob/master/debian/rules#L27-36
[2] https://cgit.freebsd.org/ports/plain/databases/lmdb/files/lmdb.pc.in
[3] https://github.com/Homebrew/homebrew-core/pull/115376#issuecomment-1310483540
[4] https://github.com/Homebrew/homebrew-core/pull/115376#issuecomment-1310462073