libsecret tarball release doesn't include meson files
The NEWS file for libsecret 0.18.8 says that meson support has been added, but the release tarball doesn't contain the meson files.
I suggest that you could add the meson build files to
EXTRA_DIST in your
Makefile.am so that tarballs generated by autotools will contain both the autotools and meson build systems until you're ready to drop autotools support.