can't configure --with-libgmp-prefix; header search order

[Ken Raeburn <raeburn@raeburn.org>]

First, a minor nit:

% ./configure --help |& head -1
`configure' configures guile 1.9.0 to adapt to many kinds of systems.
% ./configure --version | head -1
guile configure 1.9.0
%

But I downloaded 1.9.1, not 1.9.0; the version number reported here is  
wrong.

% ../configure --prefix=`pwd`/I --with-libgmp-prefix=/opt/local --with- 
libunistring-prefix=`cd ../../libunistring-0.9.1/I/&&pwd`
[...]
checking whether csqrt is usable... yes
checking how to link with libgmp... -L/opt/local/lib -lgmp
checking for __gmpz_init in -lgmp... no
configure: error: GNU MP not found, see README
%

In config.log I see that it's not using the -L option when testing for  
__gmpz_init:

configure:44666: checking how to link with libgmp
configure:45113: result: -L/opt/local/lib -lgmp
configure:45150: checking for __gmpz_init in -lgmp
configure:45185: gcc -o conftest -g -O2 -I/Users/raeburn/dev/guile/ 
libunistring-0.9.1/I/include -I/opt/local/include  conftest.c -lgmp  - 
lm -lltdl  >&5
ld: library not found for -lgmp
collect2: ld returned 1 exit status
configure:45192: $? = 1

(This is on Mac OS X 10.5.)

If instead I use CPPFLAGS and LDFLAGS on the configure command line to  
specify the /opt/local directories, configure happily runs to  
completion.  However, the build fails:

Undefined symbols:
   "_scm_cells_allocated", referenced from:
       _scm_cells_allocated$non_lazy_ptr in libtest_asmobs_la-test- 
asmobs-lib.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

This is because the build process for the test suite is picking up the  
installed Guile 1.8.7 headers (in the same MacPorts directory where  
I'm getting GNU MP from) before the guile-1.9.1 ones, as I reported  
back in June.

Ken