Proper test for POSIX_FADV_WILLNEED.

According to
http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_fadvise.html

"The posix_fadvise() function is part of the Advisory Information
option and need not be provided on all implementations."

fcntl.h's specification indeed says that these constants must be
valid in #if processing directives.

Thanks Petr Salinger for the diagnosis/fix.

git-svn-id: https://gforge.sci.utah.edu/svn/basics/trunk@295 5b8196f7-c1c0-4be6-8b13-4feed349168d
fixed large file read/writes with buffers beyond uint32_t::max() for Windows

git-svn-id: https://gforge.sci.utah.edu/svn/basics/trunk@296 5b8196f7-c1c0-4be6-8b13-4feed349168d