warnings fixes with --enable-debug

[Eric Anholt]

Here's a series for fixing almost all the warnings I get with
--enable-debug.  I had been building X without --enable-debug (and
thus without asserts) because mesa forces -O0 with --enable-debug, so
my normal config script wasn't using the flag.

Remaining warnings include the dead bswap64 in glx (need to fix mesa's
scripts), and a non-string-literal-to-sprintf warning in x86emu.

Note that in a couple of debug printfs I replaced %x with %p, which
means that the output will actually change.  I find %p output to be
much more readable, anyway.