gnome: use project compiler flags in gtkdoc
gnome: use project compiler flags in gtkdoc

If add_project_arguments is used, gtkdoc will ignore it, so make sure we
pick these flags for the compiler too.

Fixes #2901

QuLogic/meson,becm/meson,MathieuDuponchelle/meson,jpakkane/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,QuLogic/meson,becm/meson,MathieuDuponchelle/meson,jpakkane/meson,mesonbuild/meson,MathieuDuponchelle/meson,pexip/meson,MathieuDuponchelle/meson,becm/meson,MathieuDuponchelle/meson,mesonbuild/meson,becm/meson,jpakkane/meson,becm/meson,MathieuDuponchelle/meson,becm/meson,QuLogic/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,becm/meson,mesonbuild/meson,becm/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,QuLogic/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,QuLogic/meson,MathieuDuponchelle/meson,QuLogic/meson,becm/meson,pexip/meson,MathieuDuponchelle/meson,jpakkane/meson,MathieuDuponchelle/meson,QuLogic/meson,becm/meson,mesonbuild/meson,pexip/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,QuLogic/meson