Attributes GH PR #663: https://github.com/ArcticaProject/nx-libs/pull/663

nxproxy/{configure.ac,src/Makefile.am}: only enable new ELF dtags if target system actually is ELF-based.

Fixes: ArcticaProject/nx-libs#661

nxcompshad/{configure.ac,src/Makefile.am}: only enable new ELF dtags if target system actually is ELF-based.

m4/nx-macros.m4: add NX_TARGET_USE_ELF macro, setting TARGET_ELF automake conditional if target compiler is generating ELF binaries.

This reverts commit c804d99f.

It seems like the missing code was missing for a good reason. Including
it leads to very strange issues when coupled with libXcomp and using
connection speed settings lower than LAN (and even on LAN some icons
are not showing up correctly, e.g., when using MATE).

Further investigation on why this happens pending and might happen at a
later time.

Fixes: ArcticaProject/nx-libs#656

On PPC64*, X_BYTE_ORDER was hardcoded to X_BIG_ENDIAN. Since the CPUs
support both modes and we actually intend to execute nxagent on ppc64le
as well, detect the actual endianess and add support for
Ppc64LeArchitecture.

Attributes GH PR #643: https://github.com/ArcticaProject/nx-libs/pull/643

Fix XSERVER64 should be _XSERVER64

It also reintroduces a missed hack (undocumented commandline switch
-disablexineramaextension to fool clients into thinking that Xinerama
is disabled even though it is not).

That hack had been dropped in d26930d5 intentionally.

These headers have been obsoleted in glibc and are not shipped by
default any longer. Fedora follows suit.

Attributes GH PR #642: https://github.com/ArcticaProject/nx-libs/pull/642

Fixes ArcticaProject/nx-libs#640

Attributes GH PR #626: https://github.com/ArcticaProject/nx-libs/pull/626