fix meson detection of libgcrypt

The pkg-config file produced by libgcrypt is bogus as the Requires.private: gpg-error forces the use a of a pkg-config file that doesn't exist in contribs.

Alternative version !6260 (closed) where we don't fix the bogus libgcrypt.pc file but simply direct meson to use libgcrypt-config which is reliable on all systems where it's found.

The contrib name is called config-tool because that's the name meson expects in the dependency method.