Fixes ArcticaProject/nx-libs#781

Find the first available pthread library and use it together with the
-pthread flag if the C compiler is set to GCC.

Fixes: ArcticaProject/nx-libs#756

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

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.

m4/nx-macros.m4: Add NX_STRICT_OPTION and NX_DEFAULT_OPTIONS macros (borrowed from X.org's xorg-macros.m4).

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.