gnu: exim: Update to 4.93.0.4.

* gnu/packages/mail.scm (exim): Update to 4.93.0.4.
[source]: Add /fixes/ URI to the list.
[inputs]: Use gnutls/dane variant.
[native-inputs]: Add pkg-config.
[arguments]: Set USE_GNUTLS* variables to use the new inputs..
Add CC=gcc to #:make-flags.