Should be fixed by correctly building libXcomp.

See: ArcticaProject/nx-libs#756

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.

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

 Fixes ArcticaProject/nx-libs#133.