giomodule.c around line 1063:

#ifdef HAVE_CARBON
      g_nextstep_settings_backend_get_type ();
#endif

The g_nextstep_settings_backend_get_type function is prototyped in gsettingsbackendinternal.h around line 90:

#ifdef HAVE_COCOA
GType                   g_nextstep_settings_backend_get_type            (void);
#endif

and the compiling of its implementation (gnextstepsettingsbackend.c) is controlled by Makefile.am around line 132:

if OS_COCOA
settings_sources += \
	gnextstepsettingsbackend.c
endif

The function-call should be protected by testing the same token as controls the availability of the function. The implementation itself does appear to be cocoa-based, so it seems that it's giomodule.c that is the one that is incorrect.