Change alt/orig to build/to use heading levels

As the 'Alternate code' and 'Original code' heading level is now 2 the
'To build' and 'To use' subsections should be level 3, not 4, and also
reworded, this was done:

    sgit -e 's/^#### To build/### Alternate build/g' -e 's/^#### To use/### Alternate use/g' '*README.md'

Many more have to be updated and this will be done manually at another
time.

1986/wall/README.md

@@ -20,7 +20,7 @@ make all
 If you have an old compiler or a compiler that supports `-traditional-cpp` you
 might enjoy looking at the original source in [wall.alt.c](wall.alt.c).
 
-#### To build:
+### Alternate build:
 
 ```sh
 # if you have an old enough compiler: