[CM] s7_repl fails to build libc_s7.so on initial run

[Brad Christensen]

Greetings,

Upon building and running repl.c (or s7 WITH_MAIN) for the first time, I ended up in a `dumb_repl`, with the following message:

```
s7: 28-Jun-2021
load /home/pattern/bld/s7/libc_s7.so failed: /home/pattern/bld/s7/libc_s7.so: cannot open shared object file: No such file or directory
> 
```

I suppose this is expected given it being the first run. However, from reading the manual/source I also expected an attempt to automatically build libc_s7.so. Looking further into the source of `s7_repl`, it appears the loading of repl.scm never occurs due to `val` being NULL (the result of `s7_load_with_environment` failing to find and load the shared library).

Simply loading repl.scm was successful:

```
> (load "repl.scm")
writing libc_s7.c
loading libc_s7.so
(inlet ...
```

Barring any deep effort made by by `s7_load_with_environment` to find and load the shared library, I believe the problem lies in `s7_repl` as described above.

I look forward to getting to know s7 better.

Brad