[Issue 17726] Older DMD versions segfault when building Druntime with GCC 7.1

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=17726

--- Comment #2 from Vladimir Panteleev <dlang-bugzilla at thecybershadow.net> ---
Works fine without "CFLAGS += -O2", so it looks like either a GCC optimizer
bug, or UB in DMD.

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