nxagent: Also provide auto DPI feature via nx/nx option 'autodpi', taking only effect on session startups.

{nx-X11/programs/Xserver/hw/nxagent/man/nxagent.1,nxproxy/man/nxproxy.1}: magicpixel takes a boolean argument for now.

Fixes ArcticaProject/nx-libs#657.

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

Fixes: ArcticaProject/nx-libs#661

Will only be useful at build time, since installation strips any
RPATH/RUNPATH again.

Using libtool, the nxproxy binary will be replaced by a shell script
wrapper that correctly pulls in required libraries (like libXcomp), thus
making it possible to start the binary directly in the build directory.

This wrapper is dropped during installation time.

This is a C program, so enabling or disabling C++11 support will not do
anything useful.

At most it'll lead to errors when using libtool, which is the next
change to come...

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#546.

Works around libtool bug related to using both -nostdlib and -pthread
(with the former dropping latter usage.)

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.

 Fixes ArcticaProject/nx-libs#506.

nxproxy/src/Makefile.am: Actually define nxproxy_CFLAGS, so the various compiler warning flags get enabled at build time of nxproxy.

nxproxy/configure.ac: Have AC_LANG before NX_COMPILER_BRAND. Thanks to Ionic for spotting this same flaw in nxcompshad.

 This also solves the last remnant of overlinking as described in GH issue #133.

 Fixes ArcticaProject/nx-libs#133.

 Fixes ArcticaProject/nx-libs#98.