Add a check for floorl().

Also, we don't use NEED_STRPTIME_H, we use HAVE_STRPTIME.

Change-Id: I330e8ce408c05b74c50f3369691283e35e68505c
Reviewed-on: https://code.wireshark.org/review/5910
Reviewed-by: Guy Harris <guy@alum.mit.edu>