Configure for <quadmath.h>
Configure -Dusequadmath.
quadmath __float128 for longdblkind.
quadmath __float128 as NVTYPE.
quadmath needs gcc, at least 4.6.
quadmath interfaces and constants
quadmath_snprintf formats.
quadmath INSTALL.
quadmath NV formatted I/O.
quadmath Q format specifier and gcc -Wformat do not mix.
quadmath helpers export list.
quadmath has expq instead of expl.
quadmath and gconvert do not mix.
quadmath doesn't do locale radixes.
quadmath strtoflt128 accepts false infinities.
quadmath NVs cannot be cast to double etc.
darwin can have libfoo.0.dylib, as opposed to libfoo.dylib.0

Configure was changed similarly in d98292cc

cpan module; patch submitted upstream:
https://rt.cpan.org/Ticket/Display.html?id=98766
and has been applied.