ruby-fakegem.eclass: warn about using the fallback gemspec

ruby-fakegem.eclass: warn about using the fallback gemspec

The fallback gemspec does not contain dependencies so it will only
work for packages without any runtime gem dependencies. It is easy to
use it by mistake when switching from a gem to a source-based archive,
because the source-based archive does not contain the generated
metadata, but RUBY_FAKEGEM_GEMSPEC has not been set yet. This warning
alerts developers to this situation and encourages them to set
RUBY_FAKEGEM_GEMSPEC instead.

Signed-off-by: Hans de Graaff <graaff@gentoo.org>