Undefined reference to 'select()'

[walt]

Something recently broke several ports with this same link error:

foo.o(.text+0x965): In function `foo::select()':
: undefined reference to `select(int, fd_set*, fd_set*, fd_set*, timeval*)'

Firefox, fam, and musicbrainz are three examples and I thinks there are
others as well.

select() is defined in libc, and I can see with ktrace that the linker
finds libc okay -- but it can't find 'select'.

Anyone else seeing this?