My Sybase server and client are located in /usr/local/sybase. So I entered:
./configure --with-sybase-ct=/usr/local/sybase --with-apache=../apache_1.3.3 --enable-track-vars

when making apache a few steps later I received
gcc  -DSOLARIS2=260 -DUSE_HSREGEX `../apaci`   -o gen_test_char gen_test_char.o  -Lmodules/php3 -L../modules/php3 -L../../modules/php3 -lmodphp3       -L/usr/local/sybase/lib -lcs -lct -lcomn -lintl -ltcl                               -lpam -lm -ldl -lsocket -lnsl  -lresolv  -lsocket -lnsl
./gen_test_char >test_char.h
ld.so.1: ./gen_test_char: fatal: libcs.so: open failed: No such file or directory

So despite "-L/usr/local/sybase/lib" the linker could not find the database library.
In your INSTALL file you suggest the following:
"[...]If you are not on Linux and you see this error, a quick-fix is to create a symbolic
link to the shared lib from one of the directories on your link path.  Something
like 'ln -s /usr/local/lib/mysql/libmysqlclient.so /usr/lib'."

Normally you do not really want to do this. Adding /usr/local/sybase/lib into the system environment variable LD_LIBRARY_PATH worked for me.