[perl #89502] support for dld on Atari FreeMINT
fix some linuxisms and RETVAL warnings from xsubpp

Original patch by Alan, modified by Tony Cook to remove unused return
types.
freemint doesn't need the extra -lm -lposix either
new freemint hints file
add Alan Hourihane to AUTHORS
describe the level of FreeMINT support
[perl #89502] improved FreeMINT support