Make symbols generated by gdbus-codegen exportable on Visual Studio-style builds

Hi,

This updates the gdbus-codegen scripts so that the generated headers will have prototypes decorated by a $(namespace_uppercase)_EXP_GLIB_EXTERN macro (as per the suggestions in MR !1468 (merged)), which is defined to be nothing by by default, unless it is being overridden. The purpose of this is to be able to make the generated code export symbols with compiler directives where it is necessary, by overriding the definition of the $(namespace_uppercase)_EXP_GLIB_EXTERN macro.

This is demonstrated when we build the libgdbus_example_objectmanager target, where no symbols were exported on Visual Studio-style builds prior to this change set, where attempts to build documentation will break as a result of this as there is no import library which needs to be consumed by gtk-doc.

This change set are both applicable to glib master and glib-2-64 (and earlier).

With blessings, thank you!