RFC: Using -I/usr/include/gnu instead of <gnuregex.h>

I see many GNU program sources patched (and moved off the branch) just to
change <regex.h> to <gnuregex.h>

I think it can be less painful, if we leave it as <regex.h> moving the
file under /usr/include/gnu and use -I/usr/include/gnu in CFLAGS instead.

The second reason is that GNU regex evolves, and additional directories
appearse under it, like "posix". If we upgrade right now, we need to make
unneded /usr/include/(gnu)?posix, but /usr/include/gnu/posix will looks 
much better.

I plan to handle it, if no objections found.
Any comments?

-- 
Andrey Chernov | http://ache.pp.ru/