Simplify AC_LBL_CHECK_COMPILER_OPT a bit.

Rather than use -Werror=unknown-warning-option for clang, just use
-Werror. It allows to check options which are not warning options.
Use -ffloat-store while compiling if possible.

It makes life better w.r.t. GitHub bug #333. (Although the problem may persist
for unusual architectures.)
Merge pull request #334 from tejblum/float-store-2

As far as I can tell, these changes are correct and they fix GH #333.
NFS: replace ino_t with u_int32_t (GH #335 pt. 1)

NFS file handle is an opaque server-issued sequence of bytes. Parse_fh()
function implements heuristics to decode file handles generated by some
NFS servers, among other information extracting the node (inode) number.
It decodes only 32-bit node numbers.

NFS implementations use ino_t C type to represent the node number. The
type size may vary across implementations/encodings and may be missing
during compile time.

Tcpdump used to have its own typedef for ino_t. Gisle Vanem points that
it caused a problem with MSVC v.16.00.40219.01 for 80x86, which defines
the same type in <sys/types.h>. This change fixes tcpdump code to use
u_int32_t and removes the typedef.
mind MSVC's UNALIGNED macro (GH #335 pt.2)

This change addresses a compile warning that Gisle Vanem troubleshooted
in MSVC build (<winnt.h> defines UNALIGNED).
make pcap_setdirection() call conditional, GH #252

pcap_setdirection() succeeds only for particular combination of
platform, interface type and direction. Calling it on each invocation
(with PCAP_D_INOUT by default) was wrong: some interfaces are two-way,
others are one-way, and pcap_setdirection() has its own idea what it
supports and what doesn't. In particular, it was impossible to do any
capture on nflog interface in Linux since commit df7d24f. Now the
function is only called on -P flag.
README got renamed to README.ad.
That's README.md, not README.ad.
Not all compilers that can be used support C++/C99-style comments.

Use #if 0/#endif, rather than a // comment, to keep a line of code from
being compiled.
No C++/C99-style comments; not all C compilers necessarily support them.
Clean up handling of -g and -O flags.

AC_PROG_CC adds -g and -O2 to CFLAGS if you're using GCC; don't bother
adding either of them, or -O, ourselves if we're using GCC.

Add -O for all non-GCC compilers.
Note that clang, for example, is considered "gcc" by autoconf.
We require autoconf 2.61 in configure.in; that suffices.

Don't say "we require 2.50" in various macros.
With .devel, add -g3 for MIPS C and DEC C.

Lifted from the libpcap configure script, which adds -g even without
.devel, presumably so that programs linked with libpcap can get
debugging symbols.

Those compilers are for dead OSes (IRIX, Tru64 UNIX); we should probably
do whatever voodoo is needed to get Sun^WOracle C, IBM's C compiler for
AIX, and HP C to build optimized with debug symbols.
We no longer use the GCC version, so don't fetch it.

Fix some indentation while we're at it.
Update a comment for the previous commit.
Explain why we set -xansi -signed for MIPS C.
Babel: improve printing of time intervals

All Babel intervals are encoded in centiseconds and must not be 0. In an
Update TLV the interval value 0xFFFF means infinity (RFC6126 Section
4.4.9). Update the test cases.
adopt MacOS deprecation workaround from FreeRADIUS

A MacOS build of tcpdump used to produce deprecation warnings on OpenSSL
function calls. Alan DeKok explained that this is the effect of Apple's
modifications to OpenSSL and that the very same problem has already been
solved in FreeRADIUS server source code with pre-processor macros.

Copy the macros into tcpdump header file and make use of them around the
functions that call OpenSSL functions.
print-tcp: separate tcp_seq_hash into ipv4 and ipv6 versions

This cleans up and removes some duplicate code. The ipv4 and ipv6 versions of
the codepath are virtually identical now save for the variable types.