need help building and installing in non-standard place

[Gary Aitken]

I'm sure this has been addressed already, but I couldn't find what I needed
searching the archives.

I'm trying to build gimp-2.8.2 on freebsd,
and am doing it step-wise from the individual dependent packages.

What I tried (among other things...):
  unpack in /usr/home/foo/gimp_2_8/work
  cd glib-2.32.4
  ./configure --prefix=/usr/home/foo/gimp_2_8

The problem I'm having is that I want the packages I'm building installed in
one place (/usr/home/foo/gimp_2_8) but in order to configure, the configure 
script needs access to the stuff at the default sysinstall place (/usr/local)
as well, and it needs to generate makefiles that do the same.

The above ./configure fails:
  configure: error: *** No iconv() implementation found in C library or libiconv
  $ which iconv
  /usr/local/bin/iconv

I've also tried:
    ./configure --prefix=/usr/home/foo/gimp_2_8 --with-sysroot=/usr/local

./configure --help
  doesn't show anything that makes sense to me for this problem,
  although I'm sure it's been covered a bazillion times.

Seems like I need something like prefix which is a path, as opposed to a simple 
prefix, so it searches the prefix directory subtree and if not found there, 
tries the standard places.

I also tried using ./autogen.sh --prefix=/usr/home/foo/gimp_2_8 which got
me a little further but craps out in gmake due to a bunch of indirect includes
not resolving properly, as well as not using the Gettext facility.

clues?

Thanks,

Gary