It seems that GTK+ has been fixed
AS_COMPILER_FLAG(-Wshadow, ERROR_CFLAGS="$ERROR_CFLAGS -Wshadow")
AS_COMPILER_FLAG(-Wmissing-prototypes, ERROR_CFLAGS="$ERROR_CFLAGS -Wmissing-prototypes")
AS_COMPILER_FLAG(-Wmissing-declarations, ERROR_CFLAGS="$ERROR_CFLAGS -Wmissing-declarations")
-# Disabled because some GTK+ headers (like gtkitemfactory.h) are bugged :(
-#AS_COMPILER_FLAG(-Wstrict-prototypes, ERROR_CFLAGS="$ERROR_CFLAGS -Wstrict-prototypes")
+AS_COMPILER_FLAG(-Wstrict-prototypes, ERROR_CFLAGS="$ERROR_CFLAGS -Wstrict-prototypes")
AC_SUBST(ERROR_CFLAGS)