gdk_posix: disable fadvise on Linux
gdk_posix: disable fadvise on Linux

Fixes
gdk_posix.mx:747: error: 'POSIX_FADV_DONTNEED' undeclared (first use in
this function)

Debian has posix_fadvise, but for the constants to exist, one has to
define __USE_XOPEN2K.  Configure probably finds it because it ignores
the missing declaration warning from the compiler.  We could define
__USE_XOPEN2K when including fcntl.h, but probably fadvise is like
madvise, and hence like the latter, we ignore the existence of the
former.

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