[m-rev.] For review: fix extras/curs linking issue

[Fabrice Nicol]

As a follow-up to my previous email, now regarding extras/curs.

The linking issue is the same: current distributions link to both 
libncurses and libtinfo.

Rather than specifying this manually, the better option seems to run 
`ncurses6-config --libs`.

Fabrice
[This was about extras/ncurses]
>
> On most Linux platforms, linking to libncurses now also requires 
> linking to libtinfo.
>
> This can be easily checked by running 'ncurses6-config --libs'.
>
> I thought it helpful to modify Mmakefile and sample/Mmakefile to 
> mention this.
>
> Also, for users with less experience of the building process, I added 
> a couple of lines of building advice in the README, as building code 
> in 'extras' requires user intervention.
>
> A note on MGNUCFLAGS: this is not the first time that I've noticed the 
> following gcc error message on running 'mmake' (here in 
> extras/curses/sample/ but I've run into such issues elsewhere):
>
>     mgnuc --grade asm_fast.gc    -I..    --      -c smalltest.c -o 
> smalltest.o
>     gcc: erreur: option « -- » non reconnue sur la ligne de commande
>     [error: option '--' not reckognized on command line]
>
> This is caused by the fact that the mgnuc script inserts '-I..' 
> before  '--' while it apparently should do this after.
>
> Copying the command line by hand with '-I..' after '--' fixes the issue.
>
> I'm mentioning this as a possible mgnuc bug, but it may be that I 
> missed something too.
>
> For the proposed patch, commenting out MGNUCFLAGS in sample/Mmakefile 
> 'just worked'.
>
> Fabrice
>
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