Created attachment 440270 [details, diff] add libopts use flag to net-libs/gnutls gnutls can be built/used without sys-devel/autogen (needed for some libopts work). So adding a libopts USE flag can make this dependency optionnal
Bug 498324 seems to be related.
Related but different, the bug 498324 is for splitting libopts from autogen. My bug is for do not use it at all.
You cannot just modify USE without actual change in build, it suggests software will break or compile differently if library exists or not. This will cause using internal implementation of the library, which may introduce security issues. Regardless, please see >3.4 in which there is tools USE flag that is supported by upstream.