If building mrxvt you have only the option to use the default setting, which has been set with mrxvt-0.5.1, to build with rounded tabs.
If you want to use squared tabs there is no use flag in the current mrxvt ebuild 0.5.4 to enable squared tabs.
Created attachment 284003 [details, diff]
patch for squared tabs
This package is now orphan:
This is minor fix. Are you still interested?