It turns out that GDC-5 will not build; the file ...
gcc/d/d-lang.cc ... fails on this line ... 3c59416a gcc/d/d-lang.cc 2016-10-29 00:51:04 +0200 1517)debug_hooks->register_main_translation_unit (global_context);
... because register_main_translation_unit is not a member of gcc_debug_hooks.
I've found out that this member was first seen in GCC-5.2.0. Note: I'm now attempting to build GDC-5.2.0.