configure: report if `asciidoctor` was found

[jpellegrini]

Hi @egallesio ! I'm not an expert in autotools, so maybe there's a more elegant way to do this. Anyway, it would be nice to tell the user in the configure summary if asciidoctor was not found...

It won't complain or say anything if ruby-rouge is not installed, but it's already better than not saying anything at all...

SUMMARY
*******
               System:  Linux-6.5.0-1-amd64
              OS nick:  LINUX_6_5
              OS type:  unix
       Install prefix:  /usr/local
           C compiler:  gcc
    Compilation flags:  -g -O2
               Loader:  ld
       Thread support:  pthreads
     Case sensitivity:  true (by default)
Control fx parameters:  yes
System libraries used:  libffi libpcre2 libgmp libgc
   Compiled libraries:
          asciidoctor:  no (documentation will NOT be rebuilt)

[egallesio]

Hi @jpellegrini ,

Merged this one. I have changed the message (I prefer to talk about documentation, instead of asciidoctor, since this could be meaningless for some) . However, I'm not really happy with the current formulation. I f you have a better idea , feel free to reopen this PR. Thanks