I was wondering if there are any tools to present the compile time options/ flags of a package in Gentoo when emerging. I am referring to something like this
equery u CATEGORY/PKG will give you the USE flags for
CATEGORY/PKG, it will show the flags available (and to be used according to your USE settings) as well as the flags actually used for any packages already installed. Useful if you need to remember what was used for an installed package.
If you then run the command
euse -E FLAG or
euse -D FLAG you can enable or disable FLAG globally. As mentinoed in another comment
flaggie can adjust
/etc/portage/package.use, but you shouldn't need to touch that often; you would want to avoid adding something a hundred times that could just be set globally.
Take a look at app-portage/ufed.
emerge -av ... and it will display you a list of packages to be merged, including the USE flags.
To set the USE flags for a package either use
/etc/portage/package.use or a tool like
flaggie that does the job for you.