libxl: Don't optimize debug builds, and add macro debugging info

libxl debug builds are built with optimization level -O1, inherited from
the CFLAGS definition in StdGNU.mk.   Optimizations confuse the debugger,
and the comment justifying -O1 in StdGNU.mk should not apply for a
userspace library.   Disable optimization by appending -O0 to CFLAGS,
which overrides the -O1 flag specified earlier.

Also specify -g3, to add macro debugging information.   This is useful
as libxl uses many non-trivial macros.

Signed-off-by: Euan Harris <euan.harris@citrix.com>