> [...] a package [...]

   Not much meat on that bone.

> [...] I need to have termios.h for the build. [...]

   Do you need the header file, or do you need it and the run-time
library support (tcgetattr(), tcsetattr(), ...) which it implies?  Or do
you need a better "configure" script, which would be smart enough to
cope when the termios package is unavailable?

      http://en.wikibooks.org/wiki/Serial_Programming/termios

> I've gathered that I could copy over termios.h from linux, but I'm not
> sure where the system include files should go.

   Probably a waste of time, but you could put it anywhere the compiler
will find it.  Among the existing "a package" header files would be a
plausible start.

> I would be grateful for any advice.

   Useful advice would be easier to generate if you offered some clue as
to what "a package" might be, and/or what interactions with a terminal
it might use.  Information resembles many other commodities, in that
often one must give some to get some.

   In general, the terminal-using programs which I've seen adapted to
VMS have had VMS-specific terminal-handling code added.  The most common
function is disabling/enabling echo, but some programs want to sense the
size, and a small minority really wants to change the speed.  Examples
abound: UnZip+Zip, GnuPG, Kermit, ...

   Iv VMS were UNIX, then you wouldn't need to "port" an application. 
It's not, so, sometimes, some actual work is required to adapt a foreign
application to it.  Terminal handling is one of the less UNIX-like
regions of the RTL.