There is an strange automake warning

    Makefile.vapigen:49: warning: $(1) was already defined in condition TRUE, which includes condition ENABLE_VAPIGEN ...
    Makefile.am:4:   'Makefile.vapigen' included from here
    Makefile.glib:124: ... '$(1)' previously defined here
    Makefile.am:1:   'Makefile.glib' included from here

when having

    if ENABLE_VAPIGEN
    include Makefile.vapigen
    endif

That is worked around by removing the "if", which however
requires us to remove the error check in Makefile.vapigen.