How to add directory to CFLAGS (or CXXFLAGS) search path?

[Patrick Doyle]

Hello,
I am very new to developing autotools maintained projects, but like (a
lot) what I've read so far.  So, I would like to use autotools on
projects going forward, but have a large body of legacy code to
maintain.  I would like to reference an include directory from some of
that legacy code in my new, autotools maintained, program.

What is the best way to do this?  Is there an existing macro I can use
to say "Search the filesystem for somedir/custominclude.h and add
-Isomedir to CFLAGS"?

I know I can write a custom macro, or just plain m4sh code myself, to
do this (once I learn a little more), but I figured I should ask if
this has already been done.

Thanks for any pointers you care to give me...

--wpd