configure error: Cannot find ltdl.h

[david at tubits.com]

I'm trying to build the latest 1.9 code (from the svn trunk) on FreeBSD 5.4-STABLE but I'm getting a strange configure error:

> ./configure --enable-error-on-warning --enable-compile-warnings 

--enable-debug --enable-ofx
checking for a BSD-compatible install... /usr/bin/install -c
{snip}
checking ltdl.h usability... no
checking ltdl.h presence... no
checking for ltdl.h... no
configure: error: Cannot find ltdl.h -- libtool-devel not installed?

>

While there isn't a port or package that I can find called "libtool-devel", google suggests installing libltdl15, for which the pkg-list includes "include/ltdl.h".  Looks good so far, but, I *have* installed libltdl15.  Furthermore:

> locate ltdl.h

/usr/local/include/ltdl.h
/usr/local/share/libtool13/libltdl/ltdl.h
/usr/local/share/libtool15/libltdl/ltdl.h

>
and still the same configure error above.

It appears like the ltdl.h file is right where it's supposed to be, but configure doesn't see it.  I even tried adding --includedir=/usr/local/include but there's no difference.

What am I missing??

;-David