build: Remove use of GNOME_MAINTAINER_MODE_DEFINES
build: Remove use of GNOME_MAINTAINER_MODE_DEFINES

It is completely deprecated.

MathieuDuponchelle/uhttpmock,MathieuDuponchelle/uhttpmock,pwithnall/uhttpmock,Distrotech/uhttpmock,pwithnall/uhttpmock,Distrotech/uhttpmock,MathieuDuponchelle/uhttpmock