libreadlne: create symlinks for .so

libreadlne: create symlinks for .so

Currently, libreadline only installs

```
 /usr/lib/libhistory.so.8 -> libhistory.so.8.2
 /usr/lib/libhistory.so.8.2
 /usr/lib/libreadline.so.8 -> libreadline.so.8.2
 /usr/lib/libreadline.so.8.2
```

But there is no `libreadline.so` or `libhistory.so` available.

So this happens:

```
root@OpenWRT:~# cat a.c
int main() {
}
root@OpenWRT:~# gcc a.c -lreadline
/usr/bin/ld: cannot find -lreadline: No such file or directory
collect2: error: ld returned 1 exit status
```

Unless, of course, one uses `-l:libreadline.so.8`... But that
doesn't help with binaries that try to dynamically open

`libreadline.so`. I have one of those here (the STklos Scheme
compiler -- I didn't make a PR for it because it's far from
being ready, but one issue is that it does use dlopen to use
readline...)

With the symlink, it works:

```
root@OpenWRT:~# ln -s /usr/lib/libreadline.so.8 /usr/lib/libreadline.so
root@OpenWRT:~#
root@OpenWRT:~# gcc a.c -lreadline
root@OpenWRT:~#
```

Another example: when trying to package rlwrap, the build failed
complaining it could not find readline (using `-lreadline`).
It would then be necessary to change rlwrap's `configure.ac`
(and also in all packages that use readline), but it seems
simpler to add the symlinks...

This PR changes the Makefile so it will include the links.

Signed-off-by: Jeronimo Pellegrini <j_p@aleph0.info>
Link: https://github.com/openwrt/openwrt/pull/16445
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit 0000ba6ab8062f965d8fed240b76d36f9a493b55)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>