build for servers, with cllipboard related dependencies as optional

[setop]

ldd jless
        linux-vdso.so.1 (0x00007fffd170f000)
        libxcb.so.1 => /lib/x86_64-linux-gnu/libxcb.so.1 (0x00007f9035b08000)
        libxcb-render.so.0 => /lib/x86_64-linux-gnu/libxcb-render.so.0 (0x00007f9035afa000)
        libxcb-shape.so.0 => /lib/x86_64-linux-gnu/libxcb-shape.so.0 (0x00007f9035af5000)
        libxcb-xfixes.so.0 => not found
        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f9035ad5000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f903561f000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f9035b38000)
        libXau.so.6 => /lib/x86_64-linux-gnu/libXau.so.6 (0x00007f9035ace000)
        libXdmcp.so.6 => /lib/x86_64-linux-gnu/libXdmcp.so.6 (0x00007f9035400000)
        libbsd.so.0 => /lib/x86_64-linux-gnu/libbsd.so.0 (0x00007f9035609000)
        libmd.so.0 => /lib/x86_64-linux-gnu/libmd.so.0 (0x00007f90353f3000)

I can't install libxcb* on my server. I'll be glad to use jless without clipboard support.

[bew]

There are a few PRs for this already, like mine #121