GinPageIs* don't actually return a boolean

Hi,

#define GinPageIsLeaf(page)    ( GinPageGetOpaque(page)->flags & GIN_LEAF )
#define GinPageIsData(page)    ( GinPageGetOpaque(page)->flags & GIN_DATA )
#define GinPageIsList(page)    ( GinPageGetOpaque(page)->flags & GIN_LIST )
...

These macros don't actually return a boolean that's comparable with our
true/false. That doesn't strike me as a good idea.

If there's actually a boolean type defined by some included header (in
which case we don't overwrite it in c.h!) this actually can lead to
tests failing. If e.g. stdbool.h is included in c.h the tests fail with
gcc-4.9.

I think we should add a !! to these macros to make sure it's an actual
boolean.

This has been the case since gin's initial commit in 8a3631f8d86cdd9b0 .

Greetings,

Andres Freund