The new way is 'multiarch' where you just install the standard target arch packages like
zlib1g:mips
That puts the files into
/usr/include/<triplet> (or sometimes just /usr/include if they don't actually vary with arch)
Obviously your toolchain needs to know to look in the right place for
headers and libs (A modern multiarch-aware one will try both
locations, an older one will only look in the /usr/<triplet> location)