[lmi] 64-bit unit tests that succeed (fail) with gcc 8.3 (7.3)

[Greg Chicares]

All lmi unit tests succeed with
  i686-w64-mingw32-gcc   7.3
  i686-w64-mingw32-gcc   8.3
  x86_64-w64-mingw32-gcc 8.3
but a few fail with 64-bit gcc-7, and conversion between
double and std::string seems to be the problem:

numeric_io_test
  25 test errors detected; 453 tests succeeded

value_cast_test
  2 test errors detected; 190 tests succeeded

I don't plan to look into this: gcc-7.3 and 32-bit msw
are both obsolete, and it's likely that the cause is
some defect in 64-bit gcc-7 that will never be fixed.