SOLUTION: do not configure spandsp with the --prefix=/usr option, as the asterisk makefile has /usr/local hardcoded into it.  Do run these two commands to add /usr/local/lib to your library search path:

echo "/usr/local/lib" > /etc/ld.so.conf.d/usr_local.conf
ldconfig

(This caused the compile command below to go away, but I still get a crash when running the rxfax command)

===========================================================================

make[1]: Entering directory `/usr/src/asterisk-1.2.9.1/apps'
gcc  -pipe  -Wall -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -g3  -Iinclude -I../include -D_REENTRANT -D_GNU_SOURCE  -O6 -march=i686 -DZAPTEL_OPTIMIZATIONS         -fomit-frame-pointer  -fPIC   -c -o app_rxfax.o app_rxfax.c
app_rxfax.c: In function `phase_e_handler':
app_rxfax.c:105: error: structure has no member named `column_resolution'
app_rxfax.c:105: error: structure has no member named `row_resolution'
app_rxfax.c:116: error: structure has no member named `row_resolution'
app_rxfax.c:122: error: structure has no member named `row_resolution'
app_rxfax.c: In function `phase_d_handler':
app_rxfax.c:147: error: structure has no member named `columns'
app_rxfax.c:147: error: structure has no member named `rows'
app_rxfax.c:148: error: structure has no member named `column_resolution'
app_rxfax.c:148: error: structure has no member named `row_resolution'
app_rxfax.c: In function `rxfax_exec':
app_rxfax.c:281: warning: passing arg 1 of `fax_init' from incompatible pointer type
app_rxfax.c:281: error: too many arguments to function `fax_init'
app_rxfax.c:304: warning: passing arg 1 of `fax_rx' from incompatible pointer type
app_rxfax.c:307: warning: passing arg 1 of `fax_tx' from incompatible pointer type
app_rxfax.c:344: warning: passing arg 1 of `fax_release' from incompatible pointer type
app_rxfax.c: At top level:
app_rxfax.c:81: warning: 't30_flush' defined but not used
make[1]: *** [app_rxfax.o] Error 1
make[1]: Leaving directory `/usr/src/asterisk-1.2.9.1/apps'
make: *** [subdirs] Error 1
[root@sip-test asterisk-1.2.9.1]#