media-libs/nas-1.9.2 needs bison and flex to build but it doesn't have them listed as a build-time dependancy.
It'll fail with:
/bin/sh: flex: command not found
/bin/sh: bison: command not found
There are plenty of bugs for other packages with the same issue (380651, 382583, 380019, etc).
deps to bison and flex added to 1.9.3-r1 which is going stable as we speak