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.

1988/applin/README.md

@@ -26,7 +26,7 @@ program.  See the [Alternate code](#alternate-code) section below for details.
 As noted above a smaller version can be used in case your `cpp` has a hard time
 with this entry.
 
-#### To build:
+### Alternate build:
 
 ```sh
 make alt

1988/isaak/README.md

@@ -37,7 +37,7 @@ using that macro for `#define`.
 This version will not likely work on modern systems if you can even get it to
 compile.
 
-#### To build:
+### Alternate build:
 
 ```sh
 make alt

1988/litmaath/README.md

@@ -31,7 +31,7 @@ This alternate code, added to help understand the entry and for fun, is code
 that we suggested at the time of the entry publication but which we never put in
 a file.
 
-#### To build:
+### Alternate build:
 
 
 ```sh