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.

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

Needs more sophisticated features to actually test stuff correctly.

nxcomp/{configure.ac,Log.cpp}: implement configure-time std::put_time check and use macro value in Log.cpp.

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.

nxcomp/configure.ac: Don't rely on pkg-config alone when it comes to testing for presence of libjpeg shared lib and header files.

 Especially systems still using the IJG libjpeg implementation
 are likely to lack the libjpeg.pc file.
 .
 So this adds some alternative way of detecting if all libjpeg related
 build-dependencies are in place.

nxcomp/configure.ac,m4/nx-macros.m4,nxcomp/src/Socket.h: Use AC_CHECK_TYPES to detect if type 'in_addr_t' is available.

nxcomp/configure.ac: Re-add previously available configure options --with-valgrind and --with-info as renamed options --enable-valgrind and --enable-debug.