[LyX/master] Reduce the amount of needed boost headers

[Yuriy Skalko]

> On Wednesday, December 9, 2020 7:26:45 PM WET Jean-Marc Lasgouttes wrote:
>> I aml not sure we can do much more than that. The next possibility would
>> be to get rid of lexical_cast, used for convert<>(). On could use the
>> stream operator >> instead.
>> 
>> JMarc
> 
> 
> I am just curious but if/when we change to Qt 6 we can get rid of the included 
> boost dir, no?
> 
> I am asking this because, if I am reading it right, Qt 6 requires C++17.
> 
> This is an idle question so there is no need to answer. :-)
> -- 
> José Abílio

Here are current Boost dependencies:

boost/any.hpp
boost/assert.hpp
boost/crc.hpp
boost/lexical_cast.hpp
boost/signals2/signal.hpp

Only boost/any.hpp will be unused with C++17 compilers. Maybe we can get 
rid of boost/lexical_cast.hpp as Jean-Marc suggested.

Then we will still have assert, crc and signal. I haven't looked at 
assert, but for the rest we can check these alternatives:

crc --> https://github.com/d-bahr/CRCpp (one 65KB header)
signal --> https://github.com/fr00b0/nod (one 25KB header)


Yuriy