Final effect is:
make -f RubyMakefile.mk build
make[1]: Entering directory `/users/frankie/debian/debian-gis/gdal/gdal-1.5.0/swig/ruby'
/bin/sh /users/frankie/debian/debian-gis/gdal/gdal-1.5.0/libtool --mode=link g++ -fPIC -DPIC -L/usr/lib -L/usr/lib -lgeos_c -I/usr/include -lsqlite3 -lodbc -lodbcinst -L/usr/lib -lexpat -L/usr/lib -lxerces-c -lpthread -ljasper -lhdf5 -lmfhdf -ldf -logdi -lgif -ljpeg -ltiff -lpng -lnetcdf -lpq -L/usr/lib -lpq -lz -lpthread -lm -lrt -ldl -lcurl -lgssapi_krb5 -L/usr/lib/mysql -lmysqlclient -L/users/frankie/debian/debian-gis/gdal/gdal-1.5.0 -lgdal1.5.0 -lruby1.8 gdal_wrap.o -o gdal.so
libtool: link: g++ -fPIC -DPIC -I/usr/include gdal_wrap.o -o .libs/gdal.so -L/usr/lib -L/usr/lib/mysql -L/users/frankie/debian/debian-gis/gdal/gdal-1.5.0 /users/frankie/debian/debian-gis/gdal/gdal-1.5.0/.libs/libgdal1.5.0.so /usr/lib/libgeos_c.so /usr/lib/libgeos.so /usr/lib/libsqlite3.so /usr/lib/libodbc.so /usr/lib/libodbcinst.so /usr/lib/libltdl.so /usr/lib/libexpat.so -lxerces-c /usr/lib/libjasper.so /usr/lib/libhdf5.so -lmfhdf -ldf -logdi -lgif /usr/lib/libtiff.so -lc /usr/lib/libjpeg.so -lpng -lnetcdf -lpq -lrt -ldl /usr/lib/libcurl-gnutls.so /usr/lib/libidn.so -lldap /usr/lib/libgnutls.so /usr/lib/libtasn1.so /usr/lib/libgcrypt.so /usr/lib/libgpg-error.so -lgssapi_krb5 /usr/lib/libmysqlclient.so -lm -lpthread -lcrypt -lnsl -lz -lruby1.8
/usr/lib/gcc/i486-linux-gnu/4.2.3/../../../../lib/crt1.o: In function `_start':
(.text+0x18): undefined reference to `main'
collect2: ld returned 1 exit status
make[1]: *** [gdal.so] Error 1
make[1]: Leaving directory `/users/frankie/debian/debian-gis/gdal/gdal-1.5.0/swig/ruby'
make: *** [build] Error 2
Please ignore -fPIC -DPIC additional flags, they are around as an obsolete patch for 1.4.4. (see my other report about that).
This issue is solved by updating local libtool (i.e. ltmain.sh) to a more recent version. I used that currently distributed on Debian sid, 1.5.24-2 which is
VERSION="1.5.24 Debian 1.5.24-2"
TIMESTAMP=" (1.1220.2.456 2007/06/24 02:25:32)"
It also solves the other issue about missing PIC definitions.