Long Doubles Aren't Supported Correctly

While fixing issue #49, I found that the `long double` type is actually supported by glibc's libm, and gcc on 64-bit platforms actually emulates 80-bit floats for it (source: https://gcc.gnu.org/onlinedocs/gcc-4.5.3/gcc/i386-and-x86_002d64-Options.html). Investigation needs to be done into whether it's feasible to support long double's in Herbgrind.