These variables exist in Compass to control the level of browser support you're
looking to hit. These will default to true (the same as Compass), which will
output the same CSS as the regular Font Awesome CSS.
See: http://compass-style.org/reference/compass/support/
It's likely that large websites are using these variables already, so this will
help cut down on CSS that's already not required by a Compass project using
these variables.
These config vars allow fine grained control over the output of the border and
size styles. You can now set the variables to null or false in your own project
to not output this CSS.
I'm pretty unfamiliar with the Bootstrap framework, so I'm following along with
the existing comments.
The goal here is to remove and CSS when a project is not using Bootstrap
already.