byteswap.h exists only on glibc, so building libvncserver with websockets
support was not possible in other systems.

Replace the inclusion of byteswap.h and the WS_* definitions with calls to
htobeNN, which should perform the same conversions, be more portable and
avoid the need to check for the platform's endianness.

libresolv is only present on systems which use glibc; platforms such as
FreeBSD have __b64_ntop as part of libc itself.

Improve the detection process and only link against libresolv if it exists
on the system, and remember to reset CMAKE_REQUIRED_LIBRARIES after
performing the necessary tests, since we do not always want to link against
libresolv.

The current definitions were mostly useful to glibc and followed its
feature_test_macros(3) documentation.

However, this means other platforms still had problems when building with
strict compilation flags. _BSD_SOURCE, for example, is only recognized by
glibc, and other platforms sometimes need _XOPEN_SOURCE instead, or even the
removal of some definitions (such as the outdate _POSIX_SOURCE one).

_POSIX_SOURCE also had to be conditionally defined in some places, as what
it enables or disables during compilation varies across systems.

Building with -ansi failed due to some code (as well as system
headers) using non-C89 features. Fix that by adding the usual
_POSIX_SOURCE and _BSD_SOURCE definitions already present in some
other files.

Using C++-style comments when building the code with -ansi does not
work, so be more conservative with the comment style.

build_dir/rfb is not passed as an include directory automatically to
the compiler, so including that file fails.

So far, libgcrypt was looked for but no targets linked against it
directly; this caused linking problems for the client and server
examples, as the symbols they needed were not passed to the linker.

The issue that the GnuTLS websockets code uses libgcrypt regardless of
whether it has been found or not has not been touched by this commit,
though.

As requested only those lines are indented that have been changed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Signed-off-by: Rostislav Lisovy <lisovy@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

The various other headers include windows.h and the winsock headers
which give an error when SOCKET and socklen_t are already defined.

Needed for the IPv6 stuff.

These check for optional modules.

TightPNG replaces the ZLIB stuff int Tight encoding with PNG. It still
uses JPEG rects as well. Theoretically, we could build TightPNG with only
libpng and libjpeg - without zlib - but libpng depends on zlib, so this is
kinda moot.

This also fixes a compiler warning.