Make nxproxy and nxagent print an arbitrary version number by running
make NX_VERSION_CUSTOM="myvers". nxagent and nxproxy will then show
this in addition to the original version number:

NXPROXY - Version myvers (3.5.99.22)
NXAGENT - Version myvers (3.5.99.22)

Fixes ArcticaProject/nx-libs#606

nxcomp{,shad}/configure.ac: replace versionating non-portable sed construct with hopefully more portable awk construct.

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

Automake older than version 1.11 does not support dist-xz. No workaround
possible via a feature check, as far as I can tell.

Fixes ArcticaProject/nx-libs#530

nx{comp{,shad},proxy}: add configure flag --enable-cxx11 to enable and setup optional C++11 support.

Use AX_CXX_COMPILE_STDCXX(_11) from autotools archive.

Note: should only be used in tandem. Make sure that each component
either uses the C++11 ABI or none uses it. Mixing and matching libraries
and binaries with different C++ ABIs might lead to weird errors.