Improve configure option --with-libgpg-error-prefix

Note that this is not a complete solution. The libgpg-error include
directory has now a higher preference but ld may not pick up the right
library if another one is installed. The problem is that the -L
option and the -l options are not emitted separately by
gpg-error-config.

• GnuPG-bug-id: T1561