Compiling with MPFR - documentation

[Peter Lindgren]

I have been periodically compiling the latest gawk from git on a Raspberry Pi 
(under Raspberry Pi OS which is basically Debian with a custom look and feel), 
but I only recently realized that I wasn't getting MPFR support.

I googled and read the manual and looked around and wondered how to get it. 
There's a good bit in the manual about what it is, and what you can do with it, 
and how to tell if it's there, but I couldn't find anything about how to get 
it. Make option? Configure option? 

Eventually, I just guessed that gawk might use it if it were present. So I did 
some more flailing about to determine which packages to install (like searching 
for "debian buster mpfr package"), assuming I would need the development 
packages, and I eventually did this :

sudo apt install libgmp-dev libmpfr-dev

I then git pulled and rebuilt gawk as always and voila! The MPFR and GMP 
support was there. 

After the fact, I found this article by Arnold Robbins: 
https://www.linuxjournal.com/content/gnu-awk-41-teaching-old-bird-some-new-tricks-part-ii
which stated it clearly: 

"This is an optional feature: if you have the MPFR and GMP libraries installed 
when you configure and build gawk, gawk automatically will be able to use them."

Enhancement request: that this clear statement and which packages to install 
gets added to the manual. 

P.S: I really like the 5^4^3^2 example.