Add missing alt subsections to 1995/vanschnitz

The spoiler.c was also renamed to vanschnitz.alt.c and the Makefile was
updated so that running make alt will compile it. The Makefile also now
has -Wno-implicit-int.

1995/vanschnitz/README.md

@@ -4,6 +4,9 @@
 make all
 ```
 
+The authors provided a spoiler version, originally uuencoded but uudecoded by us
+in 2023. See [Alternate code](#alternate-code) below.
+
 
 ### Bugs and (Mis)features:
 
@@ -34,11 +37,23 @@ make clobber all LEVEL=6
 ## Alternate code:
 
 The authors provided a spoiler version of the program. Originally uuencoded we
-have decoded it for the wider audience in [spoiler.c](spoiler.c). The
+have decoded it for the wider audience in [vanschnitz.alt.c](vanschnitz.alt.c). The
 `uuencode`d blob is still available in their remarks below. They encourage you
 to first try and figure out the program without the spoiler.
 
 
+### Alternate build:
+
+```sh
+make alt
+```
+
+
+### Alternate use:
+
+Use `vanschnitz.alt` as you would `vanschnitz` above.
+
+
 ## Judges' remarks:
 
 Back in 1995, values of `LEVEL>8` caused some compilers (and in one case a