New --with-libgcrypt-libdir option

[Ludovic Courtès]

So far there was only a --with-libgcrypt-prefix configure option, and
from that prefix we would derive libgcrypt’s libdir to be $prefix/lib.

However, that didn’t work with the multi-arch layout used on Debian and
derivatives, where libdir is $prefix/x86_64-linux-gnu/lib (or similar.)

To address that, commit 14af289 adds --with-libgcrypt-libdir.  People
using these distros can use that flag to point to the right place.

Let me know if it works for you!

Ludo’.