[lmi] PATCH: better way to suppress gcc7 warnings in Boost headers

[Vadim Zeitlin]

 Hello again,

 Since the switch to gcc 7, we started getting "implicit fallthrough" and
"register" warnings in Boost.Regex headers that we use. You've fixed them
in 372e0735dde7e70b95e0867fbfa91540aaf4932c by disabling these warnings
globally and then improved it in 87cd20655127319bd0176d3ecc780035bfccf123,
which disabled the warnings only for Boost code, but I think this is still
not the best way to do it and we should use the same workaround as we had
already used for "shadow" warnings with gcc 6, i.e. just disable these
warnings temporarily for Boost code only using pragmas in boost_regex.hpp.

 I've created https://github.com/vadz/lmi/pull/73 doing it is very simple
and so, hopefully, could be accepted, but please notice that I did _not_
test it using the official makefiles with MinGW as I didn't set up gcc 7 in
my VM yet (BTW, do you think it's worth keeping gcc 6 in it or is it not
needed at all any longer?), but I did test it in a cross-build under Linux
and I think it should work for you too.

 Thanks in advance,
VZ