Reported by Manolis Ragkousis <manolis837@gmail.com>.
Fixes a regression introduced in 14af289e.

Before this commit, ./configure would leave LIBGCRYPT_PREFIX and
LIBGCRYPT_LIBDIR undefined, leading to LIBGCRYPT_LIBS="-L -lgcrypt" and
thus to a link failure.

* configure.ac: Make sure $LIBGCRYPT_PREFIX and $LIBGCRYPT_LIBDIR are
  never empty.