Please use pkg-config instead of libgcrypt-config to locate libgcrypt

[bzed]

Forwarding Debian Bug#1070905:

Please switch from using libgcrypt-config to pkg-config to locate
libgcrypt. libgcrypt-config will be dropped in the next libgcrypt major
release.

(Please do not switch from collectd's handwritten detection code to
AM_PATH_LIBGCRYPT() unless you invoke AM_PATH_GPG_ERROR() first. The
former rcurrently requires the latter to work without libgcrypt-config.
Just go for pkg-config instead.)

[eero-t]

Debian bug tracker does not link any patch: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1070905

Could you provide a PR for this?