Improve build.rs for gtk3-sys
Improve build.rs for gtk3-sys

gkoz/rust-gnome-glib,Susurrus/gtk,LordSentox/gtk,gkoz/rust-gnome-cairo,nicokoch/glib,rust-gnome/glib,rtsuk/cairo,EPashkin/rust-gnome-gtk,Susurrus/gtk,rust-gnome/gtk,gkoz/gtk-rs-gtk,gkoz/gtk-rs-glib,gkoz/gtk-rs-cairo,gkoz/gtk-rs-pango,gkoz/gtk-rs-gtk,EPashkin/rust-gnome-gtk,rust-gnome/cairo,gkoz/rust-gnome-gtk,rust-gnome/gtk,insanebits/gtk,drewet/gtk,daa84/gtk,EPashkin/rust-gnome-glib,daa84/gtk,LordSentox/gtk,gkoz/rust-gnome-gtk,mitaa/gtk,Susurrus/glib,mitaa/gtk,insanebits/gtk,drewet/gtk