If you haven't removed the -O1 option applied to QBVH, then the bug is not there because optimizations are disabled, not because it has been fixed in gcc.
no I commented the whole line, so qbvh was compiled with the default options (-O3 ...). I specifically checked the compile command, set cmake to be verbose, etc... Furthermore with flto flags, the real optimization is performed at the end link (you must pass -flto -Oxx when linking)
Listing all compiler (and especially unofficial gcc variant) will not be easy, finding old gcc will be tricky. I guess that setting and supporting a minimal version would be easier (I guess the oldest still alive is on mac with "apple" gcc 4.2.x). Does the bug also affect windows ? setting -O1 optimize for size on windows, "roughly" equivalent to Os which is a higher optimization level than -O1 on gcc world..