Add -lsocket for libcmsys.a build on SunOS

Solaris 11 requires explicit linking of -lsocket for some of the symbols in SystemInformation.cxx to be resolved. But perhaps there was somewhere else I should have put that to be a link library for all targets by default, but instead I put it at the lowest granularity for the need I found (as a target_link_library of libcmsys.a).

This has been tested in the context of compiling cmake.

This issue appears to have started after cmake 3.5.2 (I ran into it when compiling 3.7.1).