pgsql: Don't try to call posix_fadvise() unless supplies a

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql:
        configure (r1.495 -> r1.496)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.495&r2=1.496)
        configure.in (r1.466 -> r1.467)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.466&r2=1.467)
    pgsql/src/backend/access/transam:
        xlog.c (r1.239 -> r1.240)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.239&r2=1.240)
    pgsql/src/include:
        pg_config.h.in (r1.97 -> r1.98)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pg_config.h.in.diff?r1=1.97&r2=1.98)