Emulating "include" in gawk?

[fork]

Is there an idiom or convention for emulating an include statement, wherein
outside library code is brought into a gawk program?

I know it isn't posix, but the two features that hold me back from using awk all
the time are the lack of a library system and the lack of a strict option (like
"set -u" in shell).  But maybe there are somewhat standard workarounds.

Oh -- I don't want to use another different system that I have to install and
track like "igawk" or whatever it is.  

Thanks!