search path for AC_CHECK_HEADERS?

[mcmahill]

How can I specify a search path to try and find a 3rd party header in?

I'm trying to add a --enable-dmalloc option to a program which will cause
configure to search for dmalloc.h and link with -ldmalloc for debugging.
I'll need a similar thing to look for libs for -ldmalloc and also for a
--enable-electricfence option.

any suggestions are appreciated.

Thanks

-Dan