gio: fix compilation without F_{S,G}ETPIPE_SZ
gio: fix compilation without F_{S,G}ETPIPE_SZ

Commit a5778ef7c51044147fe470ea1707dd297f44f880 broke compilation on
architectures without F_SETPIPE_SZ and F_GETPIPE_SZ such as or1k.
If those variables are undefined, put back previous behavior, buffer
size set to 1024 * 64

Fixes:
 - http://autobuild.buildroot.net/results/398490e07343a931b25ca6ab5c90a75d7a073e9f

(Modified by Philip Withnall <withnall@endlessm.com> to add an
explanatory comment.)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

https://bugzilla.gnome.org/show_bug.cgi?id=795133

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib