Don't try to call posix_fadvise() unless <fcntl.h> supplies a declaration

Don't try to call posix_fadvise() unless <fcntl.h> supplies a declaration
for it.  Hopefully will fix core dump evidenced by some buildfarm members
since fadvise patch went in.  The actual definition of the function is not
ABI-compatible with compiler's default assumption in the absence of any
declaration, so it's clearly unsafe to try to call it without seeing a
declaration.