portability: protect O_NOATIME
portability: protect O_NOATIME

The O_NOATIME open() flag is only available on Linux, so unbreak building
by protecting it with an ifdef.

rvu95/vte,pjm0616/vte,flaxandteal/gasket-vte,thestinger/vte-ng,jessevdk/vte,pjm0616/vte,saitoha/vte-sixel,rvu95/vte,jessevdk/vte,rvu95/vte,jessevdk/vte,rvu95/vte,saitoha/vte-sixel,jessevdk/vte,thestinger/vte-ng,flaxandteal/gasket-vte,thestinger/vte-ng,flaxandteal/gasket-vte,saitoha/vte-sixel,pjm0616/vte,flaxandteal/gasket-vte