build: Use AC_USE_SYSTEM_EXTENSIONS
build: Use AC_USE_SYSTEM_EXTENSIONS

... instead of some grep magic.

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