[lmi] MinGW-w64 gcc-7.2 -Wconversion warning in wx

[Greg Chicares]

I'll turn this warning off for now with '-Wno-conversion', because
it's triggered here in the version of wx we're currently using:

/opt/lmi/local/include/wx-3.1/wx/gdicmn.h:322:21: error: conversion to 'float' 
from 'int' may alter its value [-Werror=conversion]
         { x = (int)(x*xscale); y = (int)(y*yscale); return *this; }
                     ^
/opt/lmi/local/include/wx-3.1/wx/gdicmn.h:322:42: error: conversion to 'float' 
from 'int' may alter its value [-Werror=conversion]
         { x = (int)(x*xscale); y = (int)(y*yscale); return *this; }
                                          ^

It would be nice to fix that (if it hasn't already been fixed in
a more recent version) because this warning is triggered by some
code in lmi (which I haven't investigated yet).