+=head3 quadmath
+
+One option for long doubles is that gcc 4.6 and later have a library
+called quadmath, which implements the IEEE 754 quadruple precision
+(128-bit, 113 bits of mantissa) floating point numbers. The library
+works at least on x86 and ia64 platforms. It may be part of your gcc
+installation, or you may need to install it separately.
+
+With "Configure -Dusequadmath" you can try enabling its use, but note
+the compiler dependency, you may need to also add "-Dcc=...".
+This option also turns on -Duselongdouble. At C level the type is called
+C<__float128> (note, not "long double"), but Perl source knows it as NV.
+