configure.in, network plugin: Use all tests to decide about gcrypt support.

configure.in, network plugin: Use all tests to decide about gcrypt support.

On Doug's OpenBSD machine I've seen:
 configure: gcrypt CPPFLAGS: -I/usr/local/include  -I/usr/local/include
 checking gcrypt.h usability... yes
 checking gcrypt.h presence... yes
 checking for gcrypt.h... yes
 configure: gcrypt LDFLAGS: -L/usr/local/lib
 checking for gcry_md_hash_buffer in -lgcrypt... no

Because so far the network plugin only used the existence of the header
file to determine whether or not to enable `gcrypt' support, the build
fails. With this patch, the plugin will use a define that's only
available if *all* tests were successful.