meson: Check for gcrypt with pkg-config first
Some distributions patch libgcrypt to provide a pkg-config file, and disable libgcrypt-config. One of these distributions is Yocto, which we use as the basis for the GNOME Continuous build environment.
We should check if there's a pkg-config file available for gcrypt, and use it, falling back to libgcrypt-config if the pkg-config file is not found.