Explicitly set "update = checkout" option for submodules

It seems that older Git versions don't update submodules when issuing
just "git submodule update" if this option is not set.

We might actually prefer to use "update = rebase" but for now use the
smallest change that will (hopefully) allow people using older Git
versions to get the submodule updates.

.gitmodules

@@ -1,23 +1,29 @@
 [submodule "3rdparty/catch"]
 	path = 3rdparty/catch
 	url = https://github.com/wxWidgets/Catch.git
+	update = checkout
 [submodule "src/zlib"]
 	path = src/zlib
 	url = https://github.com/wxWidgets/zlib.git
 	branch = wx
+	update = checkout
 [submodule "src/png"]
 	path = src/png
 	url = https://github.com/wxWidgets/libpng.git
 	branch = wx
+	update = checkout
 [submodule "src/expat"]
 	path = src/expat
 	url = https://github.com/wxWidgets/libexpat.git
 	branch = wx
+	update = checkout
 [submodule "src/tiff"]
 	path = src/tiff
 	url = https://github.com/wxWidgets/libtiff.git
 	branch = wx
+	update = checkout
 [submodule "src/jpeg"]
 	path = src/jpeg
 	url = https://github.com/wxWidgets/libjpeg-turbo.git
 	branch = wx
+	update = checkout