Remove distprep

Per discussion at the unconference[0], I started to write a patch that 
removes the make distprep target.  A more detailed explanation of the 
rationale is also in the patch.

It needs some polishing around the edges, but I wanted to put it out 
here to get things moving and avoid duplicate work.

One thing in particular that isn't clear right now is how "make world" 
should behave if the documentation tools are not found.  Maybe we should 
make a build option, like "--with-docs", to mirror the meson behavior.

[0]: 
https://wiki.postgresql.org/wiki/PgCon_2023_Developer_Unconference#Build_System