Put back unconditional downloading of the GTK+3 bundle

in `cross-build-mingw.sh`. Also re-arrange the code a little bit
to make it clearer what it's doing.

scripts/cross-build-mingw.sh

@@ -42,13 +42,6 @@ EOF
 done
 shift $((OPTIND - 1))
 
-if [ "$GTK3" = yes ]; then
-  BUNDLE_ZIP="$GTK3_BUNDLE_ZIP"
-else
-  BUNDLE_ZIP="$GTK2_BUNDLE_ZIP"
-  CONFIGUREFLAGS="$CONFIGUREFLAGS --enable-gtk2=yes"
-fi
-
 # USAGE: fetch_and_unzip URL DEST_PREFIX
 fetch_and_unzip()
 {
@@ -88,7 +81,15 @@ cd "$BUILDDIR"
 
 mkdir _deps
 
-fetch_and_unzip "$BUNDLE_ZIP" _deps
+# both the GTK2 and GTK3 build require files from the GTK3 bundle
+# so download and unzip it unconditionally
+fetch_and_unzip "$GTK3_BUNDLE_ZIP" _deps
+
+if [ "$GTK3" = no ]; then
+  fetch_and_unzip "$GTK2_BUNDLE_ZIP" _deps
+  CONFIGUREFLAGS="$CONFIGUREFLAGS --enable-gtk2=yes"
+fi
+
 # fixup the prefix= in the pkg-config files
 sed -i "s%^\(prefix=\).*$%\1$PWD/_deps%" _deps/lib/pkgconfig/*.pc