[m-rev.] Fix extras/curses building issues

[Fabrice Nicol]

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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