Update^5: Fix PR other/44034

[Joseph S. Myers]

This patch is OK from a doc maintainer perspective (and the C front-end 
bits are OK).  It needs middle-end and build-system review.

The comments at the start of target.def should explain the macro 
DEFHOOK_UNDOC.  They should also make clear which cases are obsolescent 
and should not be used for new hooks but are used for existing hooks 
pending appropriate license approvals from the FSF and the writing of 
documentation for hooks currently lacking it (as I understand it, all 
cases involving _UNDOC, "*" or "" for documentation or the presence of a 
comment above a DEFHOOK* macro call).

New generator files such as genhooks.c should be listed in po/EXCLUDES so 
they don't get messages extracted from them for translation.

genhooks.c has an unchecked fopen call; just because a previous call to 
fopen for the same file succeeded doesn't mean a new one will.

-- 
Joseph S. Myers
joseph@codesourcery.com