[LyX/master] Update to boost 1.72

[Neven Sajko]

> Do we still need boost included in our tree instead of just using the system one?
> IIRC we needed this for particular version like decade ago and since then there
> was no problem (correct me if I am wrong). Handling it now seems like useless
> waste of time & we polute each of our tarballs by +20 megs of sources.

For what it is worth, Archlinux does not use the boost Lyx includes in
releases anyway:

https://git.archlinux.org/svntogit/packages.git/tree/trunk/PKGBUILD?h=packages/lyx#n34