builds/zos/* portability files to z/OS UNIX

builds/zos includes:
    README.md: Overview of z/OS UNIX System Services port (Markdown)
    makelibzmq: Compile src/*.cpp and make libzmq.a
    maketests:  Compile tests/*.cpp and make test_* executables
    runtests:   Run tests/test_* executables and report results
    makeclean:  Remove built files
    zc++: /bin/c++ wrapper supplying required build arguments
    cxxall: run zc++ for all *.cpp files in directory

    platform.hpp: pre-generated (and edited) src/platform.hpp for z/OS
    test_fork.cpp: updated tests/test_fork.cpp that completes on z/OS
Extend z/OS UNIX port to allow building DLL

Updated:
   README.md:  describes process of building/using DLL
   makelibzmq: Build DLL as well as static library (unless BUILD_DLL=false)
   maketests:  Dynamically link to ../src/libzmq.so if present
   runtests:   Explicitly place ../src at start of LIBPATH
   makeclean:  Also remove files created for DLL
   cxxall:     Bumped updated date to reflect last edit
Merge pull request #1136 from ewen-naos-nz/zos

z/OS UNIX System Services port