Drop unused #define
Add macro to disable gcc attributes in public headers

MATLAB parses C headers with a Perl script that is confused by gcc
attributes.  If OPENSLIDE_SIMPLIFY_HEADERS is defined, avoid attaching
gcc attributes to function declarations.

In order to actually use this feature, MATLAB bindings will need to
create a wrapper header like this:

#define OPENSLIDE_SIMPLIFY_HEADERS
#include <openslide.h>

because MATLAB doesn't directly provide a way to define a macro before
parsing a header.
openslide.h: Omit prefetch poison under OPENSLIDE_SIMPLIFY_HEADERS

Some theoretical language binding might parse the library header and
then try to access every symbol declared in it.  Such a binding would
choke on the prefetch functions, since they are meant to throw link
errors.  Hide these prototypes if OPENSLIDE_SIMPLIFY_HEADERS is defined.
test: Report filenames for leaked FDs on Windows and Mac OS