The following is used by the autoconf:

AC_CHECK_HEADER(readline/readline.h,,AC_MSG_ERROR([readline missing]))
AC_CHECK_HEADER(readline/history.h,,AC_MSG_ERROR([readline missing]))
AC_SEARCH_LIBS(readline,readline,,[AC_MSG_ERROR([readline missing])])

What locations are used by these headers in Slackware? If it's /usr/include/readline.h, we have a slight problem

These headers are exactly in this way in my "configure.in" file.
But where can I see the locations that are used for ?

The exact msg that I have after configure is is :
checking for readline/history.h... yes
checking for library containing readline ... no
configure: error : readline missing

So that means -lreadline doesn't work on your system, but at least the headers are there.

It's (now) possible to ./configure --disable-omshell-terminal if you don't care about that OpenModelica client (it's the only one using readline).
Otherwise, I'm sure Slackware has some method of displaying what files were part of which packages. My guess is this is called -lreadline6 or something.

The following might give a better error message as to why readline won't link...

$ echo "int main() {}" > a.c && gcc -lreadline a.c

I have the foolowing msg :

root@hp:~/OPENMODELICA_1_6_0# echo "int main() {}" > a.c && gcc -lreadline a.c
/usr/lib/gcc/i486-slackware-linux/4.4.4/libreadline.so: undefined reference to `PC'
/usr/lib/gcc/i486-slackware-linux/4.4.4/libreadline.so: undefined reference to `tgetflag'
/usr/lib/gcc/i486-slackware-linux/4.4.4/libreadline.so: undefined reference to `tgetent'
/usr/lib/gcc/i486-slackware-linux/4.4.4/libreadline.so: undefined reference to `UP'
/usr/lib/gcc/i486-slackware-linux/4.4.4/libreadline.so: undefined reference to `tputs'
/usr/lib/gcc/i486-slackware-linux/4.4.4/libreadline.so: undefined reference to `tgoto'
/usr/lib/gcc/i486-slackware-linux/4.4.4/libreadline.so: undefined reference to `tgetnum'
/usr/lib/gcc/i486-slackware-linux/4.4.4/libreadline.so: undefined reference to `BC'
/usr/lib/gcc/i486-slackware-linux/4.4.4/libreadline.so: undefined reference to `tgetstr'
collect2: ld returned 1 exit status

And the ./configure command do not recognize the option : --disable-omshell-terminal

I have he following msg :
configure: WARNING: unrecognized options: --disable-omshell-terminal

You will need r8370+ of OpenModelica for this option

Anyway, it might be possible to fix this problem by adding -lcurses or -lncurses. I'd just need some time to find the correct way to do this in autoconf.