Trying to build sci-geosciences/grass-6.4.2 on a system where sys-libs/ncurses-5.9-r4 is installed with USE=tinfo, I see (among other problems, namely those addressed in bug #558226) the following:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/../../../../x86_64-pc-linux-gnu/bin/ld: OBJ.x86_64-pc-linux-gnu/curses.o: undefined reference to symbol 'raw'
/lib64/libtinfo.so.5: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
I'm not sure what made me add that USE setting in the first place, might be I can simply remove that flag and recompile everything which depends on libtinfo. However, if that were to be the official solution, then grass should depend on curses[-tinfo], I think. Otherwise, someone would have to have a closer look at the way CURSESLIB is set in configure.
I notice that the latest stable release, Grass 7.0.0, doesn't seem to mention curses in its configure.in except in a comment. I can't see any handling for libtinfo in the 6.4 release branch, so this probably hasn't been dealt with upstream. Not sure whether this is worth reporting, since it's probably gone in the latest release.
Closing as OBSOLETE - sci-geosciences/grass-7.0.1 has been added to the tree as per bug 514514 and subsequently revbumped. Note that sci-geosciences/grass:6 has been masked and will be removed from the tree in the coming weeks.