We assume all versions of MSVC have floorl().

If not, we'll have to play some games in config.nmake.

Change-Id: I1dbc5d4ef41ede0e596dbe40b4e139e0f6c39dba
Reviewed-on: https://code.wireshark.org/review/5904
Reviewed-by: Guy Harris <guy@alum.mit.edu>