Don't bother with __builtin_floorl().

At least as I read the GCC documentation, if GCC supports a builtin
floorl() at all, it will always treat floorl() and
__builtin_floorl() the same (it's reserved in C90 and defined in C99, so
nobody should ever write C code assuming floorl() won't be treated in
that afshion).

In addition, the GCC 3.3.6 manual says nothing about __builtin_floorl(),
so it probably won't help to use it.  If it appears to help, there's
probably something else going on.

Also, GCC appears not to like "#ifdef (__GNUC__)", as the parentheses
mean it's testing an expression, not a macro name.

Change-Id: Ib88b52c366d7f3b1637bb408fb18d04b67c27e4b
Reviewed-on: https://code.wireshark.org/review/5909
Reviewed-by: Guy Harris <guy@alum.mit.edu>