Using a chain of GDbus generation macros is unweildy

[Pete Woods]

THe convention in CMake for code generation is for macros to behave as follows:

set(MY_SOURCES foo.c)

generate_stuff(MY_SOURCES ........ ARGS)
generate_stuff(MY_SOURCES ........ OTHER ARGS)
generate_stuff(MY_SOURCES ........ MORE ARGS)

add_executable(hello ${MY_SOURCES})

i.e. each generator automatically appends to the MY_SOURCES variable.

See QDBus for comparison:

qt5_add_dbus_interface(
    MY_SOURCES
    "org.freedesktop.URfkill.xml"
    URfkillInterface
)

qt5_add_dbus_interface(
    MY_SOURCES
    "org.freedesktop.URfkill.Device.xml"
    URfkillDeviceInterface
)