build: Remove -L flag when $LIBGCRYPT_LIBDIR is empty.

Reported by Alex Vong <alexvong1995@gmail.com>.

* config-daemon.ac: Do not add "-L$LIBGCRYPT_LIBDIR" to LIBGCRYPT_LIBS
when "$LIBGCRYPT_LIBDIR" is empty.

config-daemon.ac

@@ -48,7 +48,7 @@ if test "x$guix_build_daemon" = "xyes"; then
   esac
 
   case "$LIBGCRYPT_LIBDIR" in
-    no)
+    no | "")
       LIBGCRYPT_LIBS="-lgcrypt"
       ;;
     *)