fquantizer patch for building with eigen 3.3.7

[jlwehle]

Building with eigen 3.3.7 runs into some problems. The platform I'm
using is FreeBSD 11.3 with the OS supplied compiler (clang version 8.0.1).

Fixed by updating include/mpreal.h with the one from eigen 3.3.7 and
then adding:

/ This macro can be used to prevent from macro expansion, e.g.:
// std::max EIGEN_NOT_A_MACRO(a,b)
#undef EIGEN_NOT_A_MACRO
#define EIGEN_NOT_A_MACRO

above:

// Fast replacement for mpfr_set_zero(x, +1):

Updated file enclosed (as a diff against /dev/null):

Note:

1. I ran into the same problem building firpm and resolved it in the
   same fashion so this same change should probably be also applied
   there.

-- John
PatchJLW02.txt