libxmlsec1: use new optional deps
Generate options immediately following depends_on

Given

  depends_on 'gnutls' => :recommended
  depends_on 'libgcrypt' unless build.without? 'gnutls'

the dependency on libgcrypt should be enabled by default. However, the
corresponding option has not yet been generated, so the condition is
true and the dependency is disabled.

Instead, add a hook method that fires after each depends_on and adds the
appropriate option.