[lmi] New clang errors fixes

[Vadim Zeitlin]

 Hello,

 I tried to build the latest sources with clang, as part of my tests, and
it turns out that there are 2 new errors: one due to the use of
non-constexpr std::ldexp() in the initialization of a constexpr variable in
bourn_cast.hpp and another one (but occurring thrice) due to the use of
deprecated std::auto_ptr<> in boost/regex.hpp.

 I've fixed both of them in https://github.com/vadz/lmi/pull/59 but I'm not
sure if we shouldn't add a helper macro for the warning-disabling pragmas,
finally: these changes double their number, bringing it up to 6, and there
probably will be need for more of them in the future. So perhaps it would
be worth having wxCLANG_WARNING_{SUPPRESS,RESTORE} macros like the ones
used in wxWidgets (which you can see at
https://github.com/wxWidgets/wxWidgets/blob/v3.1.0/include/wx/defs.h#L709-L722)?
Alternatively, we could have a wrapper header for boost/regex.hpp, but this
doesn't seem useful from a long term perspective as we know that we will
replace it with std::regex sooner or later, when it becomes not so awfully
slow.

 In any case, it would be nice if you could merge some variant of these
changes so that I don't have to apply them to my different branches in
order to test compilation with clang.

 Thanks in advance!
VZ