Guard log1p against too-old boost versions
Guard log1p against too-old boost versions

We still need to *use* log1p, in Edge3::volume, so users who actually
want quadruple precision will need boost 1.63 or better; but this should
fix anyone who includes this header (including our own test_headers)
without enabling Real==float128, plus in the future we'll want to be
able to enable float128 support inside user codes while leaving
Real==double for ourselves.

dschwen/libmesh,dschwen/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,dschwen/libmesh,libMesh/libmesh,dschwen/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,libMesh/libmesh,BalticPinguin/libmesh,dschwen/libmesh,jwpeterson/libmesh,libMesh/libmesh,libMesh/libmesh,libMesh/libmesh,libMesh/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,dschwen/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh,libMesh/libmesh,jwpeterson/libmesh,libMesh/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh