bug#36170: configure fails on FreeBSD

[Roger Mason]

Hello,

I cloned the git repo yesterday (2019-06-10, most recent commit
8d469660525d74734f3184cb9ed01b6f2dcd0445).  I ran autogen.sh in the
source directory.  I had to comment out the call to m4 --version, which
failed (I have m4-1.4.18).  I created a build directory.  When I
configure like this:

../guile/configure --with-bdw-gc=bdw-gc-threaded

it works:

---snip---
config.status: executing depfiles commands
config.status: executing libtool commands
config.status: executing po-directories commands

When I configure like this:

../guile/configure --prefix="/home/rmason/32-bit-install" 
--with-bdw-gc=bdw-gc-threaded

it fails to find various libraries & headers unless I list them
explicitly in the invocation of configure like this:

../guile/configure --prefix=/home/rmason/32-bit-install \
                   --with-bdw-gc=bdw-gc-threaded \
                   --with-libltdl-prefix=/usr/local \
                   --with-libgmp-prefix=/usr/local \
                   --with-libunistring-prefix=/usr/local \
                   --with-libiconv-prefix=/usr/local

Is that how it is supposed to work, or is there a bug in the build
system?

I note also that configure claims my readline library is too old (<
2.1), but I have:

pkg info -x readline
readline-7.0.5

Cheers,
Roger

FreeBSD 11.2-RELEASE-p10 (i386)