glib-genmarshal was rewritten in glib 2.53.4, and as of now (2.53.6)
it has a bug where it unconditionally generates marshaler bodies,
even for standard marshalers, even with --stdinc.  This causes
libgimpwidgets to define and export g_cclosure_marshal_VOID__INT()
and g_cclosure_marshal_VOID__OBJECT(), which upsets defcheck, and
breaks the build.

Work around this for now by using --header --body when generating
the marshal.c files, which includes the prototypes in the source,
instead of including the header ourselves.  This is the only code
path where the new glib-genmarshal doesn't generate bodies for
standard marshalers.  Note, however, that this usage is deprecated,
so we'll probably want to change it back once it's fixed.