As far as the discussion of GPL legalities (which should really be taken to
misc.gnu.discuss, so I won't go into depth), there are at least two
well-documented cases where linking software to GPLed code resulted in it
being GPLed in the end (by out of court settlements) 1. Next's Objective C
frontend, which is now part of GCC and 2. the FSF doesn't name the specific
software, but they say that some program that was linked against the readline
library (which is GPL) was GPL'd after the FSF found out (this is in the 'why
the LGPL is not preferable' page linked off somewhere on their license list).
Of course this is moot, because if the FSF were asked they would certainly
relicense g95's library under the GPL + exception license that other GNU
compilers use, it just shows how little Andy cares about licenses. (BTW is he
still using that multiple precision library without the required
acknowledgement? I see he says he rewrote something related to internal
arithmetic, so this might not be the case anymore.)